Preheat oven to 350 F
Spray the sides of a quarter sheet and place silpat.
In a medium bowl, whisk together the flour,BP, soal and sprinkles.
Combine the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium- high for 2-3 minutes. Scrape down the sides of the bowl, add the eggs and mix on meidum-high for 2-3 minutes.
On low speed gradually pour in the buttermilk mixture, Increase the mixer speed to med and beat for 4-6 minutes, until mixture has nearly douled in size and is completely smooth. Stop mixer and scrape sides of the bowl.
Onlow speed, gradually add the flour mixture and mix just until the batter comes together , about 1 minute. scrape down the sides of the bowl and give the batter a final mix with a rubber spatula.
Spread the cake batter in an even layer in the pan. Sprinkle the remaining tablespoons rainbow evenly on top of the batter. Bake the cake for 35 minutes. Cake should bounce back slightly and the center should no longer be jiggly. Let the cake cool.