In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cocoa powder, then set aside.
2 1/2 cups all-purpose flour, 1/2 cup cocoa powder, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
In the bowl of a stand mixer, beat the butter and sugar together on medium-high speed until light, about 3 minutes. Add the egg and egg yolk and mix well, scraping down the sides of the bowl. Add the vanilla and sour cream and beat just until combined.
1 cup granulated sugar, 1 teaspoon vanilla, 1/2 cup salted butter, 1 large egg, 1 large egg yolk, 3/4 cup sour cream
Add the dry ingredients, mixing on low just until combined, scraping down the bowl a few times to make sure the dough mixes well. Divide the dough into two sections and wrap with plastic wrap, flattening into discs. Chill at least 2 hours or more.
Preheat the oven to 425°F. Line 2 baking sheets with parchment paper and set aside.
Lightly dust a clean counter with flour, then use a rolling pin to roll out the dough to 1/4-inch thickness. Use a 2 1/2-inch round cookie cutter to cut out circles and transfer to a baking sheet.
Bake for 7-8 minutes, then immediately transfer the cookies to a wire rack to cool completely before frosting and decorating with sprinkles.
To make the frosting, beat the butter with a hand mixer until soft. Add the powdered sugar 1 cup at a time, beating well after each addition. Add the vanilla and cream, 1 tablespoon at a time, to reach the desired spreading consistency. Add the salt, then color with food coloring and use to frost cookies. Sprinkle with your favorite sprinkle mix before the frosting has a chance to set.
1/2 cup salted butter, 4 cups powdered sugar, 1 teaspoon vanilla, 2 Tablespoons heavy cream, Pinch of salt, Food coloring, Sprinkles