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

In a large mixing bowl, cream the softened unsalted butter, granulated sugar, and brown sugar on medium speed until light and fluffy.

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

In a separate bowl, whisk together all-purpose flour, baking powder, and salt. Gradually add this mixture to the wet ingredients, stirring gently.

Pour the batter into the prepared baking dish and spread evenly. Bake for 20-25 minutes until edges are golden brown and a toothpick comes out clean.

Allow the bars to cool completely in the pan before frosting with buttercream or cream cheese frosting.

Slice into generous squares and serve at gatherings or as an indulgent treat.