6 large egg whites, room temperature
1 1/2 cups sugar
1 tsp cream of tartar
1/2 cup water
1 tbsp vanilla extract
Put the egg whites in to bowl of an electric mixer or a large bowl with a hand mixer nearby.Combine sugar, cream of tartar and water in a small saucepan. Bring to a boil and cool until syrup reaches 242F (use a candy thermometer).As syrup nears correct temperature, begin to beat egg whites to soft peaks. Slowly stream in the hot sugar syrup and beat for about 5 minutes after everything has been incorporated, or until frosting reaches room temperature. Beat in vanilla until fully incorporated.
Split each cake in half and place a red half on the bottom of a cake stand or serving platter. Top with an even layer of frosting, then add a blue cake layer. Don’t worry if some squishes out of the sides when you add the next layer, as you can use this frosting to help cover the sides when the cake is fully assembled. Frost the top of the blue layer and repeat with final red and blue layers. Frost top of cake and cover sides evenly with remaining frosting.
Finish cake with red, white and blue sprinkles, as desired,