Brownie Layer: Make up brownie mix according to manufacturer directions, reserve batter.
Blondie Layers: In a small mixing bowl, mix 7.5oz sprinkles (decorettes) and 0.5oz all-purpose flour, reserve. In a stand mixer fitted with the paddle attachment, mix 3 pounds 2oz crème cake base, 10oz whole eggs, 4.35oz corn syrup, 10oz brown sugar, 0.30oz salt, and 0.30oz vanilla. Mix for 1 minute on 1st speed, then 2 minutes on 2nd speed. Scrape the bowl and then add 6.85oz butter and 2.4oz water. Mix for 1 minute on 1st speed. Scrape the bowl again and add 2.5oz of the sprinkle mixture, mix on 1st speed for 30 seconds.
To prepare, line a half sheet tray with parchment paper and treat with pan spray. Place prepared birthday cake blondie batter in a piping bag fitted with a speed icer. Pipe the batter into the sheet tray being very careful to keep the batter even and as thin as possible. This layer should weigh 2 pounds 6oz. Gently pour 1 pound 8oz prepared brownie batter on top and spread evenly to the edges. Using the speed icer, carefully pipe another layer of blondie batter on top weighing 2 pounds 6oz. Sprinkle with additional sprinkles (decorettes) and bake at 350 ͦF for roughly 40 minutes or until the internal temperature reaches 200 ͦF. Let cool completely.
Mixing the sprinkles and flour is optional, this allows the sprinkles to distribute more evenly in the batter without the colors running. This can be done in advance and stored, just make sure to mix again before using so the floured sprinkles don’t separate during storage.