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, followed by sour cream and vanilla extract until fully combined.
04 - Gradually incorporate dry ingredients into the wet mixture, mixing only until just combined.
05 - Cover dough with plastic wrap and refrigerate for at least 1 hour, or overnight for improved texture and handling.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
07 - Scoop heaping tablespoons of dough, roll into balls, and place on prepared baking sheets 2 inches apart. Flatten gently to 1/2 inch thickness.
08 - Bake for 9-11 minutes until set with lightly golden edges. Avoid overbaking for soft texture.
09 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely.
10 - Beat cream cheese and butter until creamy. Gradually add powdered sugar, mixing until smooth. Stir in vanilla and adjust consistency with milk as needed.
11 - Frost completely cooled cookies generously. Add sprinkles or colored sugar if desired.