In a large mixing bowl, combine the melted butter, light brown sugar, and regular sugar using a whisk or a hand mixer until fully combined.
To the same bowl, add in the egg and egg yolk. Whisk again until very smooth and well blended.
Pour in the dry ingredients and fold in with a spatula. Fold until just combined.
Last, add in the sprinkles and fold in until all of the sprinkles are fully incorporated.
Pour the batter into a well greased 9x9 pan, then bake at 350º for 23 minutes. Let cool completely before slicing and serving.
Notes
Do Not Over Bake - Even after the blondies are removed from the oven, the will continue to bake as they set and cool, so stick with the timeframe that is suggested in the recipe card below. The batter should be spongelike and bounce back if you gently press a finger into it.