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.