Preheat your oven to 350°F and line an 8x8 inch baking pan with parchment paper, leaving a little overhang on two sides. This makes pulling them out later super easy—trust me on this. Grease the parchment lightly with cooking spray just to be safe.

Cream together the softened butter and sugar in a large bowl until the mixture looks pale and fluffy, about 2 to 3 minutes. You'll know it's ready when it doubles in volume and looks kinda like frosting. I use my hand mixer set to medium speed because my arm always gets tired with a whisk.

Beat in the eggs one at a time, stirring well after each addition so they blend completely. Add the vanilla extract and mix for another 15 seconds. The batter should look smooth and slightly thick at this point.

In a separate small bowl, whisk together the flour, baking powder, and salt. Pour the dry ingredients into the wet mixture and stir until just combined—don't overmix or the texture gets tough. Gently fold in the whole milk until the batter reaches a thick, pourable consistency.

Fold in the white chocolate chips and about ½ cup of the rainbow sprinkles, saving some for sprinkling on top. Pour the batter into your prepared pan and spread it into an even layer. Sprinkle the remaining confetti on top so they're visible and pretty.

Bake for 25 to 30 minutes until the top is golden brown and a toothpick inserted in the center comes out with just a few moist crumbs. The center should jiggle slightly—that's how you know they're fudgy. Mine took exactly 28 minutes last time, but your oven might run a little hotter or cooler.

Cool the entire pan on a wire rack for at least 15 minutes before slicing. Cut into 9 equal squares using a sharp knife (I dip mine in hot water and wipe it between cuts for clean edges). Let them cool completely before storing, or grab one while it's still warm with a glass of cold milk.