Cream together butter and sugars for 1 minute. Add in egg, buttermilk and vanilla and mix again until smooth. Combine dry ingredients and add 1/2 to the mixing bowl, mix, then add the other half. Dough will be softer than normal cookie dough and stickier. Cover and let cool in fridge for 1 hour.
Preheat oven to 400 degrees F.
Scoop out about 2 TBSP of cookie dough onto a parchment lined cookie sheet. Bake for 9-10 minutes and let cool for an additional 2 minutes on the cookie sheet. Transfer to a cooling rack. Once fully cool, frost with about 1 TBSP of vanilla buttercream per cookie. (and sprinkles!)
TO MAKE VANILLA BUTTERCREAM FROSTING: Mix butter in a mixing bowl on medium power for about 3 minutes, until whipped and light in color. Add in vanilla, milk and 1 cup of the powdered sugar. Mix for 1 minute. Add in the other cup of powdered sugar and mix on medium for 3-4 minutes. Frost cookies.