Preheat oven to 350 degrees F and line a 9x5 loaf pan with parchment or coat with cooking spray.
In a medium bowl, whisk together flour, baking powder, salt, and milk powder (if using).
In a large bowl, whisk together sugar and eggs until slightly thickened. Add buttermilk, oil, melted butter, and vanilla extract.
Add dry ingredients to wet ingredients and mix until just combined, taking care not to over-mix.
Remove ½ cup of the batter into a separate bowl and mix with cocoa powder, sugar, and 2 tablespoon buttermilk. Stir until smooth. The batter should ribbon off the spatula; if it's too thick, add another tablespoon of buttermilk.
Add pistachio extract and green food coloring to remaining plain batter until a vibrant green (color will likely dull as loaf bakes).
Spread about ⅔ of the green batter into the bottom of prepared pan.
Top with chocolate batter in thick ribbons. And add remaining green pistachio batter over chocolate better.
Use a knife to make 3-4 s-shaped swirls through the batter.
Top batter with chopped pistachios in a thick strip down the center about 1-inch wide (bread will split as it bakes and pistachios will separate). Press lightly into batter.
Bake for 35-45 minutes, until a toothpick inserted into the center comes out mostly clean with a few moist crumbs. If top is browning too quickly, tent it with aluminum foil.
Let loaf cool in the pan for about 10 minutes before transferring to a rack to cool completely.