Preheat your oven to 350°F (175°C) and line an 8x8-inch baking dish with parchment paper.

In a mixing bowl, whisk together 1 cup of all-purpose flour, 1 teaspoon of baking powder, and a pinch of salt until well combined.

In a separate bowl, beat 1/2 cup of softened unsalted butter with 1/2 cup of brown sugar and 1/4 cup of granulated sugar for about 2-3 minutes until light and fluffy.

Add 2 large eggs and 1 teaspoon of vanilla extract to the butter-sugar mixture, beating until fully combined.

Gradually stir the dry ingredient mixture into the wet mixture until just combined, being careful not to overmix.

Gently fold in 1 cup of chopped pistachios and 1/2 cup of white chocolate chips if desired.

Pour the cookie batter into the prepared baking dish, spreading it evenly.

Bake for 25-30 minutes until the edges are lightly browned and a toothpick inserted into the center comes out with a few moist crumbs.

Allow the bars to cool in the pan for about 10-15 minutes before transferring to a wire rack to cool completely before cutting into squares.