Blend butter and vanilla in a bowl and beat until smooth for about 2 minutes. Add egg yolk and mix until incorporated well. (Preferably in a stand-up mixer with a paddle attachment)
In a separate bowl sift together all remaining dry ingredients.
Add all sifted dry ingredients to mixer with butter, vanilla and egg yolk and start mixer on lowest speed to slowly incorporate all wet and dry ingredients about 1 to 2 minutes.
Remove dough from mixing bowl, forming it into a round ball and wrap it tightly in plastic to rest in fridge for minimum of 1 hour allowing the dough to firm up so it may be rolled. (can hold dough for 3 days)
Once ready to bake cookies pull from fridge and allow to rest at room temperature for about 20 minutes. Preheat oven to 350° F.
On a lightly floured surface, with rolling pin roll dough to about a 1/4 inch thick and cut with your choice of 3- or 2-inch-round cookie cutter.
Carefully transfer cut dough to a parchment lined baking sheets about 1.5 to 2 inches apart from each other and repeat until all dough is used.
Bake for 13-15 minutes, you want them firm to the touch. Be careful not to have color on your cookies, this means you have over cooked them.
Once form remove from oven and allow them to completely cool before filling.
Spread Dulce de Leche with a knife or piping bag as preferred on one of the cookies.