Preheat your oven to 350°.
Butter and flour a 9x13x2 metal baking pan or 2 (9-inch) round cake pans and line with parchment paper.
Add the funfetti mix to the bowl of your stand mixer fitted with the paddle attachment.
In a 4-cup measuring cup, whisk the egg whites, milk and vanilla.
With the mixer on low-speed, slowly pour the wet ingredients into the dry. Mix until just combined.
Add sprinkles and gently fold them in with a rubber spatula
Pour cake batter into the prepared pan and bake on the middle rack of you preheated oven for 30 to 35 minutes (20 to 25 minutes for for two 9-inch pans or 17 to 20 minutes cupcakes) or until a cake tester comes out clean.
Allow the cake to cool in the pan for 20 minutes before running a dull knife around the edge of the cake in pan to release.
Invert the cake onto a wire rack, remove parchement and cool completely. Once cooled, invert it back into the cake pan and frost with vanilla buttercream or frosting of your choice.
Top with sprinkles before cutting and serving.