Preheat the oven to 350°F (175°C). Grease a 10- or 12-cup bundt pan with butter and dust with cocoa powder.

Whisk together flour, cocoa, 1 3/4 cups sugar, baking soda, baking powder, and salt in a large bowl. Add buttermilk, oil, 2 eggs, and 2 tsp vanilla. Beat until smooth, then stir in hot coffee until just combined.

In another bowl, beat cream cheese, peanut butter, and 1/3 cup sugar until smooth. Beat in the remaining egg and 1 tsp vanilla. Transfer to a piping bag.

Pour half the chocolate batter into the pan. Pipe a ring of filling over it, then top with remaining batter. Swirl 3–4 times with a knife.

Bake for 50–55 minutes, until a toothpick comes out with moist crumbs. Cool in the pan 10 minutes, then invert onto a rack to cool completely.

Heat cream until simmering, pour over chocolate and salt, let sit 2 minutes, then stir smooth. Pour over the cooled cake, let set 30 minutes, and serve.