In a small mixing bowl, whisk together the sour cream, egg, egg yolk, and vanilla until smooth and no streaks of sour cream remain.
In the bowl of your stand mixer fitted with the paddle attachment, mix the flour, salt, and baking powder on low speed until homogenous.
Add the butter and mix until the dough is sandy and no visible chunks remain.
Add the sugar and mix until incorporated.
Pour in the egg mixture in a slow stream and continue to mix on low speed until the dough comes together into a solid mass, 1 to 2 minutes.
Remove the dough from the bowl, form it into a thick disk, and wrap airtight.
Chill for 1 hour or up to a week before using. In the freezer, this will last for up to 3 months. To use, simply defrost in the refrigerator overnight.