Preheat oven to 350F. Butter two 9-inch round cake pans, line bottoms with parchment, then butter the parchment and dust with cocoa powder, tapping out excess.

Spread pistachios on a rimmed baking sheet and toast for 6-8 minutes until fragrant and slightly darker. Let cool completely, then roughly chop and set aside.

In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt until no lumps remain.

In a large bowl with a mixer on medium speed, beat butter and sugar until light and fluffy, about 3 minutes. Scrape down bowl as needed.

Beat in eggs one at a time, mixing well after each addition. Add vanilla and beat until combined. The mixture should look smooth and slightly mousse-like.

Reduce mixer to low. Add flour mixture in three additions, alternating with buttermilk in two additions, beginning and ending with flour. Mix just until no streaks remain. Fold in three-quarters of the chopped pistachios.

Divide batter evenly between prepared pans and smooth tops. Bake 30-35 minutes until a toothpick inserted in center comes out with a few moist crumbs. Cool in pans 15 minutes, then turn out onto wire racks to cool completely.

Place chopped chocolate in a heatproof bowl. Bring cream just to a simmer in a small saucepan, then pour over chocolate. Let stand 2 minutes, then whisk until smooth. Whisk in butter until glossy. Let cool until thick enough to spread, about 30 minutes.

Place one cake layer on a serving plate. Spread 3/4 cup ganache over top. Add second layer and spread remaining ganache over top and sides. Press remaining pistachios around the base of the cake and sprinkle a few on top. Let set at room temperature at least 30 minutes before slicing.