Preheat the oven to 350 degrees. Line the bottom of a 7x11 pan with parchment paper. Spray the paper and the sides of the pan with nonstick cooking spray.

In a medium mixing bowl, add in the cake mix, 1/4 cup lemon juice, melted butter, and the egg. Mix with a hand mixer until the batter comes together. It will be a thick, slightly sticky batter.

Spread the batter into the prepared baking pan. Using a spatula to spread it out evenly.

Place in the oven and bake for 25-30 minutes or until done. Place a toothpick in the center of the bar and if it comes out clean or with a few moist crumbs, they're done.

Meanwhile, in a small mixing bowl, add the remaining 1/4 cup of lemon juice. Add in the powdered sugar and whisk until there are no more lumps. Set aside. This will be the lemon glaze for the top.

While the bars are still hot, place all the marshmallows on the top of the bars. Try to keep it to one layer of marshmallows.

Place back into the oven for 2-3 minutes until the marshmallows have swelled up and become merged together.

Remove from the oven and immediately drizzle the lemon glaze on top of the marshmallows. Add on as many or as few sprinkles as you like. Do this while the glaze is still wet. Let dry. Cut and serve.