Using an electric mixer or a stand mixer with the paddle attachment, cream together the butter and the sugar until fluffy (2 - 3 minutes). Add the sour cream, eggs, and vanilla and mix until just combined.
Slowly add the flour, baking powder, baking soda, and salt, mixing between each addition and scraping down the sides of the bowl as necessary.
Scoop dough out using a medium-sized cookie scoop (typically a #40) and roll the dough into 2" balls (you may want to wet your fingers a bit for this). Place the dough balls about 2 inches apart on a lined cookie sheet. Use a glass or cup that has been dipped in flour, and flatten each ball of dough to about ¼" thick.
Bake in a preheated 350℉ oven for 12 to 15 minutes, until the edges have *just* started turning a golden brown. Let cool completely.
While the cookies are cooling, make the frosting by mixing the butter and vanilla until creamy. Slowly add the powdered sugar while continuing to mix, then add the milk 1 tablespoon at a time.
If desired, add a few drops of food coloring or flavoring, and mix until the color is evenly distributed.
When cookies are completely cooled, generously spread the frosting evenly over the cookies. Top with sprinkles.
Notes
Calories are automatically generated. For best results calculate based on your exact ingredients.