Preheat your oven to 350°F (175°C) and prepare a 13x9 inch baking pan by greasing it or lining it with parchment paper.

In a mixing bowl, cream together unsalted butter, cream cheese, and granulated sugar until smooth and creamy.

Add the egg and vanilla extract, mixing until fully combined.

In a separate bowl, whisk together baking powder, baking soda, salt, and all-purpose flour, then gradually mix into the wet ingredients.

Transfer the dough to the prepared baking pan and press evenly into the bottom.

Bake for 20-25 minutes until edges are golden and center is soft; a toothpick inserted should come out clean.

Allow the bars to cool completely in the pan on a wire rack before frosting.

For the frosting, mix together unsalted butter, cream cheese, powdered sugar, and vanilla extract until smooth.

Frost the cooled bars generously and top with assorted sprinkles.

Slice the bars into squares and serve at gatherings or enjoy at home.