Preheat oven to 350 degrees and spray the 13 x 9 pan with cooking spray.
Mix all of the cake ingredients together in a medium bowl until combined. Bake according to the package and let cool completely.
CHOCOLATE FROSTING
Beat the butter until smooth. Add in half of the sugar and mix. Add in the cocoa powder, Nutella, heavy cream, vanilla, and the rest of the sugar. Mix until combined. If it’s too thick, add 1 tbsp of cream at a time until you reach the desired consistency.
YELLOW BUTTERCREAM
This step is optional. Feel free to leave off the extra border if desired!
Beat the butter until smooth. Add in half of the powdered sugar and mix. Add in the rest of the sugar, cream, vanilla, and food coloring and mix.
Put frosting into a piping bag with a large tip. See notes for what I used.
ASSEMBLY
Spread the chocolate frosting on to the cooled chocolate cake, smoothing all the way to the edge.
Pipe the yellow frosting around the border.
Place the royal icing hot dogs and hamburgers in the center of the cake.
Add the larger sprinkle “balls/sixlets” to the frosting border. Add more sprinkle to the border and the middle of the cake.
Finish with Patriotic stars in the center of the cake.