Preheat oven to 350°F. Grease two 8-inch round pans.
Whisk flour, sugar, baking powder, and salt in a bowl.
In another bowl, mix milk, oil, eggs, and vanilla. Stir into dry ingredients until just combined. Fold in 1/2 cup sprinkles.
Divide batter between pans and bake 22–25 minutes. Cool completely.
Level one cake layer. Cut a 3-inch circle from the center leaving a 1-inch edge.
Place whole second layer on top. Fill center cavity with mini candies.
Frost top and sides with buttercream, hiding the seam. Press remaining sprinkles on the sides.
Chill 30 minutes before serving so candy stays contained until the cut.