Pre-heat the oven to 350℉/175℃. Line a large baking tray.
Slice the croissants open lengthways, to create a space to fill!
Mix the funfetti cake mix, oil and eggs until combined - a whisk then a spatula for when it gets stiff work well, but an electric mixer will work too. If you want to add more sprinkles, do it here! I add about 1/4 cup just to really make them pop.
Scoop 1-2 tablespoons of the cookie dough into each open croissant. Spread in an even layer - it will spill out the sides a little, so don't go all the way to the edges! Close the croissants, spread about 1 tbsp on top of each croissant.
Bake 15 minutes, until the cookie dough is turning gold at the edges. Remove and let cool on a wire rack. Dust with powdered sugar if using, and serve!