Grease pans. Preheat oven to 350° F. Spray two 9-inch cake pans with baking spray and line with Bake Even Strips. Set aside.
Mix dry ingredients. Whisk flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
Cream butter, sugar and vanilla. Cream butter, brown sugar, and vanilla in the bowl of an electric stand mixer on medium-high speed until light and fluffy, about 5 minutes.
Whisk in eggs. Add eggs, one at a time, beating until well combined.
Alternate dry ingredients with buttermilk. Reduce the speed to low, add the flour in three batches, and alternate with the buttermilk. The batter will look curdled, but it will become smooth.
Fold in sprinkles. Take the bowl off the mixer, add the sprinkles and gently fold through, also making sure that everything is well combined.
Bake! Divide the batter evenly between the two pans. Bake for 35 to 45 minutes, or until centers spring back and a toothpick comes out with a few crumbs. Let cool in pans for 10 minutes, then turn out and let cool completely on a wire rack.
Ice with frosting. I used my Vanilla Buttercream with 1 cup of rainbow sprinkles added to the frosting. For a special touch, separate a portion of the icing into three small bowls, dot with red, yellow and blue food coloring (one color in each bowl), fill pastry bags with the colored frosting and add dollops in your own decorative style. Sprinkle with more sprinkles while the buttercream is still moist.