In a large, microwave-safe bowl, melt the butter. (This should take about a minute and a half.) Let the butter cool down so it doesn't cook the next ingredients. Once the butter is cooled, mix in the egg, light brown sugar, and imitation vanilla. If you don't have imitation vanilla, you can use pure vanilla extract, but the imitation vanilla gives a much more yellow birthday cake-ish flavor! The imitation kind is cheap and can usually be found near the pure vanilla extract at the grocery store, so if you can use that, I'd recommend it.
In a medium-sized microwave safe bowl, combine the 5 tablespoons unsalted butter, ½ cup granulated sugar, cocoa powder, and vanilla extract. Heat on high for 45 seconds, stir, and then heat for an additional 15 seconds, or in 15 second intervals until the butter is completely melted and can be mixed with the additional ingredients easily.
Add in the egg and stir until mixture is well blended and thick. Add the flour, salt, and baking powder and mix until fully incorporated, and no lumps remain. Try not to overmix as this will make your brownies tough.
Pour the brownie layer on top of the birthday cake layer and gently smooth it out across the pan using a knife, spatula, or the back of a spoon. Pour the remaining birthday cake blondie batter on top, and smooth again. Top with additional sprinkles.
For the Birthday Cake Layers
For the Brownie Layer