In a medium size bowl combine the cocoa powder, the sugar, salt, boiled water and whisk to combine until smooth. Set aside to cool.
1/4 cup cocoa powder, unsweetened, 1/4 cup sugar, 1/4 cup boiled water, pinch of salt
In a medium bowl whisk together the flour and baking powder, and set aside.
2 ½ cups flour, regular all-purpose, 2 tsps baking powder
Place your softened butter in the bowl of your stand (or hand held) mixer and beat using the paddle attachment of your mixer until smooth, approximately 3 - 5 minutes.
¾ cup + 1 tablespoon unsalted butter, at room temperature
Add the icing sugar to the butter and beat at medium speed for 3 minutes. Add the egg and vanilla and beat together at medium speed for an additional 3 minutes.
1 cup icing sugar, 1 tsp vanilla extract, 1 egg, large
Begin adding your flour to the butter, sugar and egg mixture. Add half the flour, mix well and then add the rest of the flour; mix well until just combined. Be careful not to over mix. The dough will be a little dense, and slightly crumbly. This is okay! Transfer the dough to another mixing bowl and mix with your hands until the dough holds together.
If you have a scale, weigh your dough and separate it into two . One of the halves will become your chocolate dough. If you don't have a scale, eyeball it as much as you can.
Add one of the halves back into the mixer and slowly add in your cooled chocolate, and 1/4 cup flour. Mix until the chocolate is incorporated.
1/4 cup flour
Let all your dough rest at room temperature for 10-15 minutes.
Preheat your oven to 350 °F.
Weigh out 15 grams of vanilla dough and 15 grams of chocolate dough for each cookie. If you don't have a scale, take ½ tablespoon of each dough for each koulouraki (a total of 1 tablespoon dough per cookie). To shape your koulourakia, roll out the chocolate and the vanilla dough so that you make two long pieces, each about 2 - 3 inches long. Attach them together so that you end up with one long roll of dough that is 4 - 6 inches in length. Shape into a U shape and twist to shape your koulouraki. Transfer your cookies to a parchment lined baking pan. See Recipe Note for a simpler way to make these cookies.
Once your tray is full, brush each cookie with the egg wash, made by whisking together the egg yolk and milk.
1 egg yolk, 1 tbsp milk
Bake in the middle rack of your oven for about 20 minutes, until the tops are golden. Remove from oven and let the koulourakia cool on the baking sheet for 2 - 3 minutes before carefully transferring to a cooling rack to cool completely.
Store in a well covered container at room temperature for 5 - 7 days. Keep in refrigerator, or freezer, for longer storage.