Preheat the oven to 170°C (350°F). Grease or line the base and sides of a 20 cm (8 inches) square cake tin or baking dish with parchment paper.
Place the cake mix into a mixing bowl with the eggs, oil, vanilla, and the add-ins (if using) and mix with a wooden spoon or spatula until well combined.
Transfer the batter to the prepared tin, and use a metal spoon to spread it evenly out in the tin.
Bake for 20-25 minutes or until the edges are slightly brown and the middle has just set and is no longer wet.
Remove from the oven and place the tin on a wire rack. Leave the brownie to cool completely in the cake tin.
Remove from the tin, spread on the frosting, decorate with sprinkles, and cut it into pieces.