Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan by greasing it lightly or lining with parchment paper.

In a large mixing bowl, cream together softened unsalted butter and granulated sugar until light and fluffy. Add in the egg and vanilla extract, blending until smooth.

In a separate bowl, whisk together all-purpose flour, baking powder, and salt. Gradually incorporate this into the wet mixture until combined.

Spread the cookie dough evenly into the prepared pan and bake for 12-15 minutes until golden brown. Let cool completely.

In a medium bowl, whisk together the instant pistachio pudding mix, milk, and heavy cream until thickened, about 5-10 minutes.

Once cooled, spread the pistachio pudding over the cookie base evenly.

Sprinkle chopped pistachios on top of the pudding layer, gently pressing them in.

Refrigerate for at least 2 hours to set. Cut into squares and serve chilled.