Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper.

In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Beat in the eggs one at a time, then mix in the vanilla extract.

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

Stir in the white chocolate chips, if using. Spread the dough evenly into the prepared baking pan.

Bake for 18–22 minutes, or until the edges are lightly golden and a toothpick inserted in the center comes out clean.

Allow the cookie bars to cool completely in the pan before cutting into squares and serving.