the goods:
1/2 cup (1 stick) unsalted butter, cut into small pieces, plus more for pan
1 cup all-purpose flour, plus more for pan
1/2 teaspoon salt
1/4 teaspoon baking powder
8 ounces semisweet chocolate, coarsely chopped
1 and 1/4 cups sugar
1 teaspoon vanilla extract
2 large eggs
1/2 cup buttermilk
3/4 cup shelled, unsalted pistachios, coarsely chopped
ganache:
1/2 cup heavy cream
4 ounces semisweet chocolate, coarsely chopped
1/4 cup shelled unsalted pistachios
Preheat oven to 350 degrees. Butter and flour an 8-inch round cake pan; line bottom with parchment paper. In a medium bowl, whisk together flour, salt, and baking powder. Set aside.
In a large heatproof bowl set over (not in) a saucepan of simmering water, melt butter and chocolate, stirring frequently, 4 to 5 minutes. Remove bowl from pan. Whisk in sugar and vanilla, then eggs, buttermilk, and pistachios. Fold in flour mixture just until combined.
Pour batter into prepared pan. Bake until toothpick inserted in center comes out with a few moist crumbs attached, 55 to 60 minutes. Let cool in pan, 5 minutes; run a knife around edge, and invert onto a wire rack. Remove paper, let cool completely.
While cake is cooling, make ganache: In a small saucepan, bring cream to a simmer; remove from heat. Add chocolate, and let stand 5 minutes; whisk until smooth. Let cool until mixture falls back in ribbons when lifted with a spoon, 2 to 5 minutes.
Set cake on a serving platter. Pour ganache onto center of cake; spread evenly over the top and let run down sides. Let set, about 30 minutes. Sprinkle top with pistachios.
