Preheat your oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper for easy removal.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, using an electric mixer or whisk (about 3-4 minutes).
Add in the egg and vanilla extract, mixing well until combined.
In another bowl, whisk together the all-purpose flour, baking powder, salt, and pistachio pudding mix until evenly combined.
Gradually add the dry ingredients to the wet mixture, mixing until just combined. Do not overmix.
Spread the cookie dough into the prepared baking pan evenly using a silicone spatula.
Bake in the preheated oven for 20-25 minutes, or until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Remove from the oven and let the bars cool completely in the pan on a wire rack.
Once cooled, cut into 12 bars. If desired, sprinkle with chopped pistachios and white chocolate chips for additional topping.