Preheat the Oven: Preheat your oven to 350°F (175°C) and line a 9×13-inch baking pan with parchment paper, leaving an overhang for easy removal.

Prepare the Cookie Base: In a bowl, cream 1 cup butter and 1 cup sugar until fluffy. Add 1 egg and 1 teaspoon vanilla extract; mix well. Stir in 2 cups flour and 1/4 teaspoon salt until combined. Spread this mixture evenly in the prepared pan and bake for 12-15 minutes until lightly golden. Let it cool slightly.

Prepare the Pistachio Layer: In another bowl, cream 1 cup butter and 1 cup sugar until fluffy. Add 2 eggs, 1 teaspoon vanilla extract, and pistachio pudding mix. Mix well. Stir in 2 cups flour, 1/2 teaspoon baking powder, and 1/4 teaspoon salt. If desired, add green food coloring for a richer color.

Assemble and Bake: Spread the pistachio cookie layer over the cooled base. Bake for 25-30 minutes or until the top is set and lightly golden. A toothpick inserted into the center should come out clean.

Cool and Slice: Let the bars cool completely before lifting them out with the parchment paper. Slice into squares or bars.

Add Toppings (Optional): Garnish with crushed pistachios, drizzle with white chocolate, or dust with powdered sugar for extra flavor and presentation.