Preheat your oven to 350°F (175°C). Grease the 9x13-inch baking pan or line it with parchment paper for easier removal.
In a mixing bowl, cream together the softened butter and granulated sugar using an electric mixer or a whisk until light and fluffy (about 2-3 minutes).
Add the egg and vanilla extract to the butter-sugar mixture and beat until well combined.
In another bowl, combine the all-purpose flour, baking powder, and salt. Gradually add the dry mixture to the wet mixture, mixing just until combined.
Fold in the chopped pistachios until evenly distributed.
Spread the cookie dough evenly into the prepared baking pan, smoothing the top with a spatula.
Bake in the preheated oven for 20-25 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean.
Allow the bars to cool in the pan for about 10 minutes, then transfer them to a wire rack to cool completely.
Once cooled, cut into bars and dust with powdered sugar if desired before serving.