01 - Whisk together flour, baking soda, baking powder, and salt in a medium bowl. Set aside.
02 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
03 - Mix in the egg until incorporated, then blend in sour cream and vanilla extract until fully combined.
04 - Gradually incorporate dry ingredients into the wet mixture, mixing just until flour disappears. Do not overmix.
05 - Cover bowl with plastic wrap and refrigerate for at least 1 hour, or overnight. Chilling is essential for proper texture and handling.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
07 - Scoop heaping tablespoons of dough and roll into balls. Place on prepared sheets 2 inches apart. Gently flatten to 1/2 inch thickness.
08 - Bake for 9-11 minutes until cookies appear set and edges are lightly golden. Avoid overbaking for optimal softness.
09 - Let cookies rest on baking sheets for 5 minutes, then transfer to wire rack to cool completely before frosting.
10 - Beat cream cheese and butter until creamy. Gradually incorporate powdered sugar until smooth. Add vanilla and milk as needed for spreadable consistency.
11 - Generously frost each completely cooled cookie. Add sprinkles or colored sugar if desired.
12 - Store frosted cookies in an airtight container in the refrigerator for up to 5 days.