makes one 9" round (or 8" square) bundt
Preheat oven to 375 degrees F. Spray bundt pan with cooking spray and dust with cocoa powder, taping out excess. Put sugar, chocolate, butter and water in saucepan over medium heat. Stir occasionally 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 in the mid-section comes out clean, about 30-35 minutes. Let cake cool 10 minutes, then remove from the pan and cool on a rack.