In a large mixing bowl combine the egg yolks, sugar, lemon zest and vanilla extract. Whisk until all the ingredients are incorporated. Add soften butter and whisk again.
Add the flour, baking powder and salt and start mixing with a spatula. When the flour has absorbed all the liquid ingredients, add the sour cream and keep mixing the dough. When mixing with a spatula becomes too hard to do, start kneading the dough with your hands. The dough should turn out very soft and not stick to your hands.
To form the cookies, tear off a small piece of dough and roll into a ball the size of a walnut. Flatten it a little and place on the baking sheet covered with parchment paper or Teflon sheet. Bake it at 350 F degrees for 15-18 minutes. Let them cool completely and dust with the powdered sugar.