Spray or grease a 12-cup Bundt pan and dust with flour. Set aside. Preheat oven to 350° F.
Combine butter, cocoa powder, chopped dark chocolate, and water in a saucepan and cook over medium-low heat, stirring often, just until melted. Remove from heat and set aside.
In a large mixing bowl, combine flour, sugar, salt and baking soda and whisk together to combine. Add half of the chocolate mixture to the dries and stir well to combine. Add remaining chocolate mixture to the batter and stir until completely blended.
Add eggs, one at a time, stir well after each. Whisk in yogurt, cider vinegar, and vanilla and blend well.
Pour batter into prepared pan and bake in the center of the oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Remove from oven and cool for 15 minutes before inverting onto a cooling rack. When the cake is completely cooled, move it to the cake serving plate.
While the cake is cooling, prepare the icing and chop the toffee.