I got started on this via a recipe found on Reddit (here's a link to the gif, I have no idea why the link is named this: https://gfycat.com/CaringLavishFrenchbulldog.). This lead me to do research on the whole concept.
It turns out Milk Washing is a resurgence of an old technique for making punch, and there are a ton of suggestions about methods, ingredients, etc. Essentially the technique is to make a punch and then slowly pour the punch into the milk (NEVER the other way around). This curdles the milk; the curdles remove solids and make the liquid transparent and depending on the ingredients a pale pink color. Here's a detailed article from Cook's Illustrated: https://www.cooksillustrated.com/science/844-articles/story/the-key-to-crystal-clear-cocktails-milk-really. Here are some recipes:
Mary Rockett's Citrus Milk Punch
This recipe is from the Cook's Illustrated article. Here are some notes:
Use all the zest from the lemon and the orange. Take wide, shallow swipes of zest with a vegetable peeler, trying your best to avoid any of the unpleasantly bitter white pith that sits just below. The best coffee filter for the job is a large basket-style filter. While the brandy-milk mixture drains, don’t push or press the liquid through the filter—let it slowly drain through and you’ll get the clearest punch possible. Milk punch can be refrigerated for up to one month.
Ingredients
2 cups brandy
½ ounce lemon zest strips (from 1 lemon)
½ ounce orange zest strips (from 1 orange)
1 cup whole milk
2 cups water
½ cup plus 1 tablespoon (4 ounces) sugar
¼ cup plus 1 tablespoon lemon juice
¼ cup orange juice
Instructions
1. Combine brandy, lemon zest strips, and orange zest strips in lidded container and let sit at room temperature for at least 18 hours or up to 2 days. Discard zest strips.
2. Place milk in 8-cup liquid measuring cup or large pitcher; set aside. In large liquid measuring cup or large bowl, whisk infused brandy, water, sugar, lemon juice, and orange juice until sugar dissolves.
3. Pour brandy mixture into milk. Gently stir curds with small spoon. Let sit for at least 30 minutes or cover and refrigerate for up to 24 hours.
4. Line fine-mesh strainer with coffee filter and set over large measuring cup or bowl. Gently pour brandy-milk mixture into coffee filter and let drain. Drain strained punch mixture through curds in coffee filter one more time. Discard curds and coffee filter. Transfer clarified punch to lidded glass container and refrigerate until ready to serve. To serve, pour ¼ cup to 3/8 cup chilled punch into small glass.
Black Tea - Port Milk Punch
This is from Cook's Illustrated but is identical to the GIF recipe I link to above, except it adds an additional 4 teaspoons sugar. Here are notes from the recipe:
Don’t worry about using particularly high-end tea for this recipe. Subtleties will be lost to the other punch ingredients as well as the clarification process. Batavia Arrack is a classic punch liquor fermented from sugarcane and a small amount of rice. It is far funkier than most rums, but if you can’t find it, rum can be substituted. The best coffee filter for the job is a large basket-style filter. As the tea-milk mixture is draining, don’t push or press the liquid through the filter—let it slowly drain through and you’ll get the clearest punch possible. Milk punch can be refrigerated for up to one month.
Ingredients
1 cup whole milk
1/3 cup lemon juice
2 cups brewed black tea
2/3 cup ruby port (he used Sandeman Founders Reserve)
2/3 cup Batavia Arrack (Van Oosten is the best known importer) or rum
1/3 cup plus 4 teaspoons sugar
Instructions
1. Place milk in 8-cup liquid measuring cup or large pitcher; set aside. In 4-cup liquid measuring cup or large bowl, whisk tea, Batavia Arrack, port, sugar, and lemon juice until sugar dissolves.
2. Pour tea mixture into milk. Gently stir curds with small spoon. Let sit for at least 30 minutes or cover and refrigerate for up to 24 hours.
3. Line fine-mesh strainer with coffee filter and set over large measuring cup or bowl. Gently pour tea-milk mixture into coffee filter and let drain. Drain strained punch mixture through curds in coffee filter one more time. Discard curds and coffee filter. Transfer clarified punch to lidded glass container and refrigerate until ready to serve. To serve, pour ¼ cup to 3/8 cup chilled punch into small glass.
Billy Dawson's Milk Punch
Another one from Cook's Illustrated. Described as "spirit-forward (a real drinker's drink)". Here are notes from the recipe:
Use all the zest from the lemon. Take wide, shallow swipes of zest with a vegetable peeler, trying your best to avoid any of the unpleasantly bitter white pith that sits just below. Use an aged amber rum in this recipe. Batavia Arrack is a classic punch liquor fermented from sugarcane and a small amount of rice. It is far funkier than most rums, but if you can’t find it, rum can be substituted. The best coffee filter for the job is a large basket-style filter. When the rum-milk mixture is draining, don’t push or press the liquid through the filter—let it slowly drain through and you’ll get the clearest punch possible. Milk punch can be refrigerated for up to a month.
Ingredients
1 cup whole milk
½ cup plus 1 tablespoon (4 ounces) demerara sugar
1 ounce lemon zest strips (from 2 lemons)
1 cup boiling water
1-¼ cups rum
½ cup plus 2 tablespoons brandy
¼ cup plus 2 tablespoons porter or stout
¼ cup plus 2 tablespoons lemon juice
2 tablespoons Batavia Arrack
Instructions
1. Place milk in 8-cup liquid measuring cup or large pitcher; set aside. Combine sugar and zest strips in 4-cup liquid measuring cup or large bowl. Using back of ladle, gently muddle zest and sugar until oil releases from zest and sugar starts to look wet and clumpy, 45 to 60 seconds. Add boiling water and whisk to dissolve sugar. Whisk in rum, brandy, porter, lemon juice, and Batavia Arrack.
2. Pour rum mixture into milk. Gently stir curds with small spoon. Let sit for at least 30 minutes or cover and refrigerate for up to 24 hours.
3. Line fine-mesh strainer with coffee filter and set over large measuring cup or bowl. Gently pour rum-milk mixture into coffee filter and let drain. Drain strained punch mixture through curds in coffee filter one more time. Discard curds and coffee filter. Transfer clarified punch to lidded glass container and refrigerate until ready to serve. To serve, pour ¼ cup to 3/8 cup chilled punch into small glass.