Preheat the oven to 350F then line an 8x8 baking pan with parchment paper or foil. Set side.
In a large bowl, cream the butter, granulated sugar, and brown sugar together until smooth and creamy. Add in the egg, and the vanilla extract, and mix to combine.
1/2 cup butter, 1/2 cup granulated sugar, 2 tbsp brown sugar, 1 large egg, 2 tsp vanilla extract
Mix in the flour, baking powder, and salt just until the flour mixture barely disappears. Spread the batter into the prepared baking pan and bake in the preheated oven for 15 minutes. The center will still look raw but don't worry these bars will continue baking as they cool down.
1/2 tsp baking powder, 1 & 1/4 cup all purpose flour, 1/4 tsp salt
Frosting
In a large bowl, whip the softened butter, and powdered sugar together until combined. Add in the heavy cream and vanilla and whip until light and fluffy.
4 tbsp butter, 1 tsp vanilla exract, 2 tbsp heavy cream, 1 cup powdered sugar
If you want to give these the same marbled look as mine, divide he frosting into two bowls and color one with the pink gel food coloring and the second one with the purple food coloring.
Frost the sugar cookie bars with dollops of purple frosting and dollops of pink frosting, then top with sprinkles. slice and serve if you prefer your frosting on the softer side or chill them a bit to allow the frosting to set before enjoying.