Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

In a large mixing bowl, combine softened butter, granulated sugar, and brown sugar. Beat until light and fluffy, about 3 to 5 minutes.

Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.

In a separate bowl, whisk together flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, mixing until combined.

Pour the dough into the baking dish, spreading it evenly. Bake for 20 to 25 minutes, until edges are golden brown and the center is set.

Let the bars cool completely before frosting them with buttercream or cream cheese frosting.