Preheat oven to 350°F. Coat an 8-inch square baking pan with cooking spray. Line bottom and sides of pan with Reynolds Kitchens® Unbleached Parchment Paper, leaving a 2-inch overhang on 2 opposite sides; set aside.

Whisk together ½ cup boiling water and cocoa in a small heatproof bowl; set aside.

Whisk together brown sugar, oil, and vanilla in a large bowl until well combined. Whisk in egg, egg yolk, and buttermilk. Add baking powder, baking soda, and salt; whisk to incorporate.

Add cocoa mixture (do not stir), then add flour. Whisk vigorously until batter is well combined, about 20 seconds.

Transfer batter to prepared baking pan. Bake until a wooden pick inserted into center comes out with a few moist crumbs, about 25 minutes, rotating pan halfway through baking time.

Let cake cool completely in pan on a wire rack, about 1 hour and 30 minutes. Lift cake from pan using parchment paper overhang as handles; transfer to a platter. Discard parchment paper, and set cake aside.

Make the pistachio frosting: Beat butter, vanilla, and salt in a stand mixer fitted with the paddle attachment on medium speed until well combined, about 1 minute.

Reduce mixer speed to medium-low, and gradually beat in powdered sugar, pistachio paste, and heavy cream, stopping to scrape down sides of bowl as needed.

If desired for a brighter green color, add 2 to 4 drops of green food coloring. Increase speed to medium-high, and beat until frosting is light and fluffy, about 2 minutes.

Spread frosting evenly over cake, and garnish with chopped pistachios.