Preheat oven to 350 degrees F. Spray a 9x9-inch baking dish with nonstick spray and line with parchment paper.

Add the white chocolate chips and butter to a microwave safe bowl. Microwave for 20 seconds, stir, and then microwave for another 20 seconds. Stir until completely melted.

Pour the chocolate mixture into a larger bowl. Add the sugars and whisk to combine. Whisk in the egg, vanilla, and milk.

In a separate bowl, whisk together the flour, cake mix, and salt.

Fold the dry ingredients into the wet. When the mixture is just about combined, add in the sprinkles. Fold to combine until no more flour is visible.

Spread the batter into the prepared dish. Top with a little sprinkle of a few more sprinkles.

Bake for 20 minutes until edges are just slightly golden. Let cook completely in the pan before removing and slicing into 12-16 squares.

Once the cooled squares are ready to be served, make the buttercream frosting.

Add the butter and 1/2 cup of powdered sugar to a stand mixer fitted with the whisk attachment. Mix on low to combine and then on high to cream the butter for two minutes. Mix in the vanilla and salt.

Mix in 1/4 to 1/2 cup of powdered sugar at a time until it becomes stiff.

Spoon dollops of frosting on top of each blondie and then top with more sprinkles.