In a medium bowl, whisk together the flour, baking soda, baking powder;, salt, and cocoa powder, then set aside.
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.
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 degrees 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 ¼-inch thickness. Use a 2 ½-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.