Prepare the Dough. Cream together softened butter and granulated sugar for 2-3 minutes until light and fluffy. Add the egg and vanilla extract and mix until well combined.
Combine Dry Ingredients. In a separate bowl, whisk together the flour, baking powder, and salt.
Mix the Dough. Gradually add the dry ingredients to the butter mixture and mix until a soft dough forms.
Add Sprinkles(optional). Gently fold in the colorful sprinkles until evenly distributed throughout the dough.
Press into Pan. Press the dough evenly into a greased or parchment-lined 9x13" baking pan.
Bake to Perfection. Bake in a 350F preheated oven for 18-22 minutes until the edges are lightly golden and a toothpick inserted in the center comes out with a few moist crumbs.
Cool and Frost (Optional). Allow the bars to cool in the pan for a bit, then transfer to a wire rack to cool completely. If desired, frost the bars once they have cooled and top with additional sprinkles.