Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking dish with non-stick spray or line it with parchment paper.

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

Gradually add in the eggs and pistachio pudding mix, mixing until fully incorporated.

Whisk together the all-purpose flour and baking powder in another bowl, then slowly incorporate into the wet ingredients.

Fold in the chopped pistachios gently until evenly distributed.

Pour the batter into the prepared baking dish and spread evenly.

Bake for 25-30 minutes or until the edges are golden brown and a toothpick comes out clean.

Let cool completely in the dish before slicing into squares.