1In a medium bowl, whisk 5 cups of flour, baking soda, and baking powder; set aside.
2In the bowl of a stand mixer with the flat beater attached, cream the butter and granulated sugar at medium speed until light and fluffy, about 3 minutes. Scrape down the sides of the bowl with a rubber spatula as needed. Add the eggs, one at a time beating until each is incorporated. Add the vanilla and sour cream and beat at low speed until combined.
3Add the dry ingredients and beat at low speed until just combined, about 30 seconds, scraping down the bowl as needed. Dough needs to obtain the right consistency for rolling, so add additional flour, 1/4 cup at a time, until this is achieved (up to 1 cup more flour). Divide dough into two sections. Flatten into rectangles about 1 1/2 inches thick, then wrap with plastic wrap. Chill in the refrigerator overnight. Preheat the oven to 425 degrees. Line 2 large cookie sheets with parchment paper or spray them with nonstick cooking spray, set aside.
4Generously flour a work area and rolling pin. With a rolling pin, roll the dough out to 1/4-inch thickness. Using a 2 1/2-inch round cookie cutter, cut out circles and transfer to a baking sheet. Bake for 7-8 minutes, until pale golden. Immediately transfer cookies to a wire rack to cool.
5For the frosting:
To make the frosting, in the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and vanilla. Slowly beat in the powdered sugar. Once smooth and creamy, add in heavy cream, 1 tablespoon at a time until the desired spreading consistency is achieved.
6Once cookies have cooled completely, frost and add sprinkles. Allow frosting to set, then store in an air-tight container. Let cookies sit overnight before serving to allow the flavors to develop.