Step 1

Preheat the oven to 350 F (177C). Line a 9X5 loaf pan with parchment paper along the long sides, leaving a 1-2" overhang). Set aside.

Step 2

In a small bowl, whisk the flour, baking powder and salt together. Set aside.

Step 3

In a medium or large mixing bowl, whisk the melted butter and sugar together until full combined. Add the egg and whisk until smooth. This should take several turns around the pan. Get those arms muscles moving! Whisk in the vanilla extract.

Step 4

Add the dry mixture to the wet. Using a spoon or spatula, stir the dry ingredients into the wet until mostly combined. Add the sprinkles, and mix until fully combined. Pour the blondie batter into the prepared pan. Make sure to smooth the batter evenly into the corners and across the pan.

Step 5

Bake at 350 F (177 C) for 28-35 minutes. The blondie is finished baking when the edges are set and pull gently away from the sides of the pan. The top will be golden brown. A toothpick inserted into the center will yield no crumbs (or a very few moist). Cool in pan for 30 minutes. Use a butter knife to gently loosen the edges of the blondie that are touching the pan. Using the parchment overhangs, pull the blondies out of the pan. Cool on a cooling rack if topping with the buttercream.

Step 6

In a stand mixer, with paddle attachment, place butter, 1/2 cup powdered sugar, vanilla extract and 1 tbsp milk. Beat until smooth and creamy. On low, add 1/4 c powdered sugar at a time. If needed, add 2-3 tsp more milk, one teaspoon at a time. Once smooth and creamy, beat on high until desired frosting consistency is reached. Frosting should be thick and spreadable.Top blondie bars right before serving.