Preheat oven to 350°F and grease two 9-inch round pans with butter and cocoa powder. Line bottoms with parchment.
In a large bowl, whisk flour, sugar, cocoa, baking soda, baking powder, and salt.
In another bowl, whisk eggs, buttermilk, coffee, oil, and vanilla until smooth.
Pour wet ingredients into dry and stir just until combined. Batter will be thin.
Divide between pans and bake 28–32 minutes, until a toothpick comes out with moist crumbs.
Cool in pans 10 minutes, then transfer to racks to cool completely.
For ganache, heat cream until just simmering, then pour over chopped chocolate. Let sit 2 minutes.
Stir gently until smooth, then mix in butter and salt.
Spread ganache over cooled cakes and stack layers. Chill briefly before slicing.