Preheat oven to 350. Line the bottoms of two 6"x3" round pans with parchment paper, then grease bottom and sides. Set aside. Combine dry ingredients and set aside. Melt butter, then whisk together with sugars. Add egg, sour cream, milk, and vanilla and mix until combined. Slowly add dry ingredients and mix until no lumps remain. Add sprinkles, stir gently, being careful not to overmix. Pour evenly into prepared pans. Bake for 25-30 minutes or until cakes test done with a toothpick. Cool completely.

Using a stand mixer with paddle attachment, mix together butter, salt, lemon juice, and extract until well blended. Gradually add powdered sugar one cup at a time and whip vigorously. Gradually add milk until desired texture is reached.