Combine the flour, salt, baking powder, baking soda, and cornstarch in a bowl then whisk together and set aside.
Cream the butter and sugar until fluffy then beat in the egg, vanilla, and almond extract if using. Add the sour cream and mix until combined, scraping the bowl down as needed.
Add the dry mixture to the wet and mix on low until just combined. Use your spatula to scrape down the bowl and combine any errant flour. Shape dough into a disk then cover in plastic and chill for an hour.
Preheat oven to 375F. Roll the dough out to ¼ inch then use a 2 1/2-3 inch circular cookie cutter to cut the cookies, Reroll the scraps and cut the remaining cookies. Transfer cookies to parchment-lined baking sheets and bake at 375F for 8-9 minutes or until the cookies are just turning color on the bottom edge. Set aside to cool completely.
While the Cookies bake, cream the butter then mix in the powdered sugar, and vanilla. Drizzle in the milk while mixing on low then increase speed to high and beat until smooth. If using food dye, mix in a drop at a time until desired color is reached.
Top the cookies with a thin layer of buttercream and a smattering of sprinkles.