1/2 cup unsalted butter, melted and slightly cooled
1 cup granulated sugar
1 large egg, room temperature
1 1/2 teaspoons pure vanilla extract
1 1/2 cups all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon salt
1/3 cup rainbow sprinkles (jimmies work best)
Optional: 1/2 cup white chocolate chips
Instructions
Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving a little overhang on the sides. This makes it much easier to lift the bars out once they have cooled.
In a large mixing bowl, whisk together the melted butter and granulated sugar until well combined. The mixture should look slightly grainy but fully incorporated.
Add the egg and vanilla extract to the butter mixture. Whisk vigorously for about one minute until the batter looks smooth and slightly pale. This helps create that perfect chewy texture.
Using a spatula, gently fold in the all-purpose flour, baking powder, and salt. Stir until just combined; be careful not to overmix, as this can make the cake bars tough instead of tender.
Add the rainbow sprinkles (and white chocolate chips if using) to the batter. Gently fold them in until they are evenly distributed throughout the thick dough.
Spread the batter into your prepared baking pan, smoothing the top with your spatula. Bake for 22–25 minutes. The edges should be lightly golden, and a toothpick inserted into the center should come out with just a few moist crumbs.
Allow the bars to cool completely in the pan. This is a crucial step to ensure they set properly. Once cool, use the parchment overhang to lift the block out and slice into 16 even squares.