01 - Whisk together flour, baking soda, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
03 - Add the egg to the butter mixture, beating well. Mix in sour cream and vanilla extract until fully combined.
04 - Gradually add dry ingredients to the wet mixture, mixing just until incorporated. Do not overmix.
05 - Cover dough with plastic wrap and refrigerate for at least 1 hour or overnight for proper handling and texture.
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 baking sheets 2 inches apart. Gently flatten each ball to 1/2 inch thickness.
08 - Bake for 9-11 minutes until cookies appear set and edges are barely golden. Avoid overbaking for soft texture.
09 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely before frosting.
10 - Beat cream cheese and butter until creamy. Gradually add powdered sugar, beating until smooth. Stir in vanilla extract. Add milk as needed for spreadable consistency.
11 - Generously frost each completely cooled cookie. Add sprinkles or colored sugar if desired.