Special equipment: a 2-tablespoon ice cream scoop
For the cookie dough: Preheat the oven to 325 degrees F. Line 2 baking sheets with parchment.
Whisk the cake flour, baking powder, baking soda and salt in a medium bowl. Mix the granulated sugar, butter and sour cream in the bowl of an electric mixer on medium-high speed until smooth, light and fluffy, about 3 minutes. Add the vanilla and egg and mix until combined. Reduce the speed to low, add the flour mixture and mix until just combined, about 1 minute.
Using a 2-tablespoon ice cream scoop, drop level scoopfuls of dough onto the prepared baking sheets, making sure to leave 2 inches between each cookie. Dip the bottom of a glass cup into some cake flour. Gently press the cup on top of each mound of dough and flatten until you get a 1 1/2-inch circle, flouring the bottom of the cup as needed. Use a wet finger to smooth out any rough sides.
Bake until the cookies puff up and are pale yellow and barely starting to brown on the bottoms, about 20 minutes. Let cool on the baking sheets for 1 minute, then transfer to a wire rack to cool completely, about 1 hour.
For the frosting: Meanwhile, mix the confectioners' sugar, butter, cream, vanilla and 2 drops of the pink food coloring in the bowl of an electric mixer. Mix until light, fluffy and almost doubled in size, about 3 minutes.
Frost the cookies with the frosting and sprinkle with confetti sprinkles.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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