Preheat your oven to 350°F (175°C) and line your baking pan with parchment paper for easy removal.
In a large mixing bowl, whisk together the melted butter, dark brown sugar, and granulated sugar until well combined. Add the egg and vanilla extract, and mix until smooth and creamy.
In another bowl, whisk together the all-purpose flour, baking powder, and kosher salt.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix.
Gently fold in the sprinkles, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it out evenly. Bake for 20-25 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Allow the blondies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Cut them into squares and enjoy!