Preheat the oven to 350 degrees F. Spray a 9-by-13-inch pan with cooking spray and line with parchment paper.
Heat the butter together with the unsweetened chocolate in a medium saucepan over low heat, whisking occasionally, until fully melted and smooth. Sprinkle in the cocoa powder while whisking; whisk until smooth. Allow to cool for 5 minutes.
Stir in the sugar and vanilla until just combined. Stir in the eggs one at a time. Stir in the flour until just combined. Transfer the batter to the prepared pan, distributing it evenly.
Bake until a toothpick inserted in the middle comes out clean, 20 to 25 minutes. Allow the brownies to cool completely, then flip out onto a cutting board. Cut the brownies into bite-size pieces and transfer to a baking rack.
Put the bittersweet chocolate in a medium bowl. Heat the cream over medium-high heat until the edges begin to bubble, 2 to 3 minutes. Immediately pour the cream over the chocolate. Allow to sit for 2 minutes, then stir until smooth.
Spoon the ganache over the brownie pieces until completely covered. Sprinkle the top with chopped pistachios and allow to sit until the ganache has set, about 10 minutes. Transfer to a platter and serve with toothpicks.