01 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl. Set aside for later use.
02 - Beat softened butter and granulated sugar in a large mixing bowl until light and fluffy, approximately 2-3 minutes.
03 - Add egg and vanilla extract to the butter mixture, beating until fully incorporated. Mix in sour cream until smooth.
04 - Gradually add dry ingredients to wet mixture, mixing until soft dough forms. Divide dough in half, flatten into discs, wrap in plastic, and refrigerate for minimum 1 hour.
05 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
06 - On lightly floured surface, roll dough to 1/4-inch thickness. Cut 2.5-inch circles and place on prepared baking sheets, spacing 2 inches apart.
07 - Bake for 9-11 minutes until edges are set but not browned. Cool completely on wire rack before frosting.
08 - Beat softened butter until creamy. Gradually add sifted powdered sugar, beating well after each addition. Add vanilla, salt, and cream; beat on high 2-3 minutes until fluffy.
09 - Once cookies are completely cool, spread or pipe frosting generously on top. Add sprinkles if desired.