How to Make It
Preheat oven to 350°F. Grease and flour a 14-cup Bundt pan. In a medium sized bowl, combine flour, baking powder, salt, and sprinkles. Set aside.
Using the paddle attachment on a stand mixer, cream together butter, shortening, granulated sugar, and brown sugar on medium-high for 3 minutes. Scrape down sides of bowl, add eggs, and beat on medium high for another 2 to 3 minutes. Scrape down sides of bowl again.
Change paddle to whip attachment on stand mixer. On low speed, stream in liquid ingredients. Increase to medium-high and whip for 4 minutes, until almost very airy and fully combined.
On very low speed, add the dry ingredients. Mix until just combined, about 1 minute. Remove mixing bowl from mixer and use a spatula to scrape the bottom of the bowl and make sure everything is well incorporated.
Pour batter into prepared pan and spread around evenly.
Bake until golden brown and until a toothpick inserted comes out clean, about 40 minutes. Allow to cool completely before inverting out of pan, about 1 1/2 hours.
Icing the Cake: Measure out 1/2 cup of powdered sugar into three small mixing bowls. To one bowl, add 2 tsp. milk. To another bowl add 1 1/2 tsp. milk and red food coloring. To the third bowl, add 1 1/2 tsp. milk and blue food coloring. Mix the contents of each bowl with a clean spoon until smooth. Transfer each icing into separate piping bags or a ziplock bag. Transfer Cake to a wire rack set over a rimmed baking sheet. Starting with the thinner white icing, drizzle the icing over the cake using a back and forth motion from the outside of the cake to the inside hole, working around the entire perimeter of the cake. Repeat process with red and blue icings. Carefully transfer cake to a serving platter using two spatulas to slide underneath the cake. Decorate cake with additional sprinkles.