Prep: Preheat oven to 400°F. Line a baking sheet with a Silpat or parchment paper. Set aside.
Cream butter and sugar: In a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a hand mixer, cream together the butter and sugar until smooth and light in color.
Mix wet ingredients: Add in the sour cream, eggs, vanilla extract, and additional flavored extract, if using. Mix until just combined.
Sift and add dry ingredients: In a separate mixing bowl, sift together the flour, baking powder, baking soda, and salt. Add the dry ingredients to the wet in three additions—mixing well after each.
Roll cookies: Roll dough into 2″ balls and place 2″ apart on the prepared cookie sheet. Use a roller (a mini roller works great) to roll cookies flat (about 1/4 to 1/2 inch thick).
Bake: Bake cookies in preheated oven for 7 minutes. They will not be brown when done. Remove from oven and transfer from cookie sheet to cooling rack. Allow cookies to fully cool before frosting.