1. Preheat the Oven:

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

2. Make the Cookie Bar Dough:

• In a large bowl, cream the softened butter, granulated sugar, and brown sugar together until light and fluffy.
• Beat in the eggs, one at a time, followed by the vanilla extract.
• Add the pistachio pudding mix and mix until combined.
• In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
• Fold in the chopped pistachios, if using.

3. Bake the Bars:

• Press the dough evenly into the prepared baking dish. Bake 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 completely in the pan.

4. Make the Frosting:

• In a mixing bowl, beat the softened butter until smooth. Add the pistachio pudding mix and powdered sugar, and mix on low speed.
• Gradually add the milk, one tablespoon at a time, until the frosting reaches a spreadable consistency. If desired, add a few drops of green food coloring for a vibrant green color.

5. Frost the Bars:

• Spread the frosting evenly over the cooled cookie bars. Sprinkle with chopped pistachios for garnish, if desired.

6. Slice and Serve:

• Use the parchment paper overhang to lift the bars out of the pan. Slice into squares or rectangles and serve.