Preheat oven to 350°F (175°C) and grease an 8x8 inch square baking pan.

In a medium bowl, combine melted butter, sugar, eggs, and vanilla extract.

Stir in cocoa powder, flour, salt, and baking powder until just combined.

Spread the brownie batter into the prepared pan and bake for 25-30 minutes.

Allow brownies to cool completely in the pan.

In a separate bowl, cream together softened butter and instant pistachio pudding mix.

Gradually add powdered sugar and milk, beating until smooth and fluffy.

Add green food coloring if a more vibrant color is desired.

Spread the pistachio frosting evenly over the cooled brownies.

Sprinkle chopped pistachios over the top and chill for 30 minutes before slicing into 16 squares.