Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan or line it with parchment paper.

In a medium microwave-safe bowl, melt ½ cup of unsalted butter and let it cool slightly.

Add 1 cup of brown sugar and ½ cup of granulated sugar to the melted butter. Whisk until smooth.

Incorporate 2 eggs, one at a time, and then stir in 1 teaspoon of vanilla extract until glossy.

In a separate bowl, whisk together 1 ½ cups of all-purpose flour, 1 teaspoon of baking powder, and ½ teaspoon of salt. Gradually mix this with the wet ingredients.

Fold in ½ cup of rainbow sprinkles gently until evenly distributed.

Pour the batter into the prepared baking pan and spread evenly with a spatula.

Bake for 20–25 minutes until golden edges appear and a toothpick comes out with a few moist crumbs.

Allow to cool on a wire rack for about 10 minutes before removing from the pan and letting it cool completely before slicing.