Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the egg and vanilla extract, mixing until well combined.
In a separate bowl, whisk together the flour, baking powder, salt, and pistachio pudding mix.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chopped pistachios and white chocolate chips if using.
Spread the cookie dough evenly into the prepared baking pan.
Bake for 20-25 minutes or until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Allow to cool in the pan for at least 10 minutes before cutting into bars.