makes one 9" round (or 8" square) bundt; adapted from Amanda Hesser, Cooking for Mr. Latte
Preheat oven to 375°F. Grease bundt pan with softened butter and dust with cocoa powder (spray intricate bundt pans with baking spray and dust with cocoa powder), taping out excess. Put sugar, chocolate, butter and water in saucepan over medium heat, stir now and then until all ingredients are melted and combined. Remove from heat and let cool 15–20 minutes.
Whisk flour, leavenings, and salt in a large bowl. When the chocolate has cooled, whisk in the buttermilk, eggs and vanilla then pour over the flour mixture and whisk to combine, but don't over-mix. Pour the batter into the prepared bundt pan and bake on the middle rack until a toothpick inserted comes out clean, about 30-35 minutes.
Let cake cool 10 minutes, then invert onto a cooling rack.