Make the cookies: Cream the sugar and butter together until smooth. Add the eggs, sour cream and vanilla. In a separate bowl, sift together the flour, baking powder, baking soda and salt. Add the dry ingredients to the wet ingredients and mix well. Dough will be soft and sticky.
Divide the dough in half. Return half the dough to the mixing bowl and add red food color; mix well. Turn the red dough out onto a piece of waxed paper and wrap. Wash the mixing bowl and return the remaining dough to the bowl. Add green food color and mix well. Transfer the dough to a piece of waxed paper and wrap. Refrigerate both portions of dough until semi-firm, at least 1 hour.
Preheat the oven to 350°F. Line two baking sheets with parchment paper.
Remove cookie dough from refrigerator. Use a tablespoon to scoop the dough into portions, and roll between your palms. Lightly dust your hands and the dough with flour if sticky. Place dough balls on the cookie sheets 2inches apart. Gently flatten the dough balls with your palms. If there are streaks of flour on the cookies, gently dust it off with a soft pastry brush.
Bake the cookies for 10-12 minutes, or until they are puffed and fragrant. Transfer the cookies to a wire rack to cool completely.
Make the frosting: Cream the butter with an electric mixer; beat in the powdered sugar 1/2 cup at a time. Beat in the vanilla. If the frosting is stiff, beat in the milk or cream 1 tablespoon at a time until a smooth, fluffy consistency is achieved.
Use an offset spatula to spread the frosting onto the cookies (I used a disposable piping bag with the tip snipped to get an evenly round layer of icing, then used and offset spatula to smooth it out). Cover the icing with confetti sprinkles soon after icing.
I tested two techniques for making these cookies. The first was rolling the dough into balls and flattening them (as indicated in the recipe) which worked pretty well but was very sticky and I had to dust my hand with flour constantly. The second time I dusted a work surface with flour, dusted a rolling pin, dusted the dough, and rolled the dough to a little less than 1/2-inch thick. Then I cut the dough into 2-inch circles. Both yielded good results, but the ball-rolling seemed to go a little quicker.
Keyword american buttercream, confetti sprinkles, sour cream