Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan or line it with parchment paper.

In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Mix until well combined. Add in the eggs, one at a time, along with the vanilla extract, stirring until incorporated.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually mix the dry ingredients into the wet mixture until just combined. Avoid over-mixing!

Gently fold in the rainbow sprinkles, preserving their vibrant colors.

Pour the blondie batter into the prepared baking pan, spreading it evenly. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out mostly clean.

Let the blondies cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely. Once cool, cut into squares and enjoy!