Mix Dry Ingredients: In a bowl, combine flour, baking powder, baking soda, and salt.
Cream Butter and Sugar: In another bowl, beat unsalted, softened butter and granulated sugar until light and fluffy.
Add Wet Ingredients: Mix in one room-temperature egg, vanilla extract, and optional lemon zest for a fresh flavor.
Incorporate Sour Cream: Stir in sour cream until well combined.
Combine Mixtures: Gradually add the dry ingredients to the wet mixture, mixing until a dough forms.
Bake: Scoop 1 ½ tablespoon portions of dough onto parchment-lined baking sheets, spacing them out. Bake until light golden.
Flatten and Cool: Use the bottom of a glass to flatten the cookies for a smooth frosting surface. Let them cool completely.
Prepare Frosting: Mix buttercream ingredients. Add color or flavor, like pulverized freeze-dried strawberry powder, if desired.
Frost: Use an offset spatula to spread frosting on the cooled cookies. Add sprinkles if you like.