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

Mix the Dry Ingredients: In a spacious bowl, whisk together the flour, pistachio pudding mix, sugar, and baking powder until evenly blended.

Add Wet Ingredients: Stir in the egg, melted butter, vegetable oil, milk, and vanilla extract until a smooth batter forms.

Fold in Pistachios: Gently fold in the chopped pistachios for a burst of texture in every bite.

Bake: Transfer the batter into the prepared baking dish, spreading it out evenly. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the bars cool completely before applying the frosting.

Blend the Ingredients: In a medium bowl, beat the softened cream cheese and butter together until the mixture is smooth and creamy.

Add Vanilla and Sugar: Mix in the vanilla extract, then gradually add the powdered sugar. Beat until the frosting reaches a fluffy consistency.

Assembly: After the cookie bars have fully cooled, spread a generous layer of cream cheese frosting over the surface, smoothing it out evenly. For a festive touch, sprinkle additional chopped pistachios on top. Cut into squares and enjoy your delectable pistachio cookie bars!