Combine 2 tbsp water, 1 packet yeast and 1 tsp sugar in a small bowl and leave for about 15 minutes until the yeast puffs up and becomes activated.
Place the 3 cups sifted flour into a large bowl, then add 1 cup softened butter into the flour. Use a pastry cutter or a fork to cut the butter into the flour until it is a bit crumbly.
Add activated yeast mixture along with 3 egg yolks, 1 cup sour cream, and 1 tsp vanilla extract. Mix with an electric hand mixer until a cohesive dough forms. Do not overmix!
Cover the mixing bowl with a dish towel. Leave the dough to rest on the counter for 1 hour, then place the mixing bowl in the fridge and allow it to rest for another 4 hours. Alternatively, you can skip the counter resting and simply allow the cookie dough to rest in the fridge overnight.
Place 2 cups walnuts in a food processor and pulse until they are chopped into small pieces. In a separate, medium-sized bowl, combine the walnuts, ½ cup sugar, ¼ cup milk, and 1 ½ tsp vanilla. Mix well with a spoon and set aside.
When the dough has finished resting, preheat the oven to 375 degrees Fahrenheit.
Roll the dough to ⅛” thickness on a floured board. Cut into 3” squares.
Put 1 heaping tsp of filling in the center of each square.
Moisten the edges of the pastry with water. Fold one corner over in the center, wrapping the walnut filling. Then fold the other corner as well.
Bake on an ungreased cookie sheet for 15 minutes.
Cool and sprinkle with powdered sugar. Enjoy!