Step 1

Preheat oven to 350ºF. While preheating, be sure to read tips & tricks in post above for recipe success.

Step 2

Warm the butter in a microwave-safe bowl or saucepan until it’s just melted. If steaming, allow to cool for a few minutes before proceeding.

Step 3

In a large mixing bowl, combine the butter with the brown sugar and granulated sugar. Stir until combined. (I recommend mixing this by hand rather than a mixer.)

Step 4

Stir in eggs and vanilla extract, followed by the baking powder and salt.

Step 5

Mix in cornstarch and flour, followed by the white chocolate chips & rainbow sprinkles. Mix the dough until well-combined. This is a "wetter" dough but if it is runny, let it sit and firm up for a few minutes (or add 1-2 tablespoons more flour).

Step 6

Grease and line a 9x13 light-metal pan with parchment paper. Spread dough into an even layer in pan. If desired, press some extra sprinkles on top for added presentation.

Step 7

Bake for 28-32 minutes, or until edges are slightly golden brown. The center may look slightly underbaked but the residual heat from the pan will continue to bake the bars even after you take it out of the oven.

Step 8

Cool completely before slicing into 12-24 squares.