01 - Whisk together flour, baking soda, baking powder, and salt in a medium bowl until evenly combined. Set aside for later use.
02 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes using an electric mixer on medium speed.
03 - Mix in the egg until fully incorporated, then add sour cream and vanilla extract. Continue beating until smooth and well combined.
04 - Gradually add dry ingredients to the wet mixture, mixing on low speed just until flour disappears. Avoid overmixing to maintain tender texture.
05 - Cover bowl tightly with plastic wrap and refrigerate for minimum 1 hour or up to overnight. Chilling prevents excessive spreading and enhances flavor development.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper and set aside.
07 - Scoop heaping tablespoons of chilled dough, roll into smooth balls, and place on prepared sheets 2 inches apart. Gently press each ball to 1/2 inch thickness.
08 - Bake for 9-11 minutes until tops appear set and edges show minimal golden color. Remove immediately when slightly underbaked for optimal softness.
09 - Let cookies rest on baking sheets for 5 minutes to firm, then transfer to wire rack to cool completely before frosting.
10 - Beat cream cheese and butter until creamy and smooth. Gradually incorporate powdered sugar, then mix in vanilla extract. Add milk 1 tablespoon at a time until spreadable consistency achieved.
11 - Generously spread frosting over completely cooled cookies. Top with sprinkles or colored sugar immediately while frosting remains tacky.
12 - Transfer frosted cookies to airtight container and refrigerate up to 5 days. Separate layers with wax paper to prevent sticking.