2 cups sugar
4 ounces unsweetened chocolate
1/4 pound unsalted butter (1 stick), plus more for greasing the pan
2 cups all-purpose flour, plus more for dusting the pan
2 teaspoons baking soda
1 teaspoon baking powder
1 teaspoon semi-coarse sea salt or kosher salt
1 cup milk
1 teaspoon cider vinegar
1 teaspoon vanilla extract
1 1/2 cups Nestle semi-sweet chocolate chips
1 1/2 cups sour cream, at room temperature
Preheat the oven to 375 degrees, and place a baking sheet on the lowest rack, to catch any drips when the cake bakes. Put the sugar, unsweetened chocolate, butter and 1 cup of water in a saucepan. Place over medium heat and stir occasionally until all of the ingredients are melted and blended. Remove from the heat and let cool slightly, 15 to 20 minutes.
2. Meanwhile, sift together the flour, baking soda, baking powder and salt. In a small bowl, stir together the milk and vinegar. Grease and flour a 9-inch tube pan. (If you prefer, you can grease it, line it with parchment and then grease and flour it. This is not necessary, but parchment does make getting the cake out easier.)
3. When the chocolate in the pan has cooled a bit, whisk in the milk mixture and eggs. In several additions and without overmixing, whisk in the dry ingredients. When the mixture is smooth, add the vanilla and whisk once or twice, to blend. "Dump" the batter into the tube pan and bake on the middle rack until a skewer inserted in the center comes out clean, about 30 minutes. Let the cake cool for 10 minutes, then remove from the pan and cool on a rack. (This can be tricky, so if someone is around, enlist them to help. Place a ring of wax paper on top of the cake so you have something to grab onto when turning it out.) Let cool completely.
4. Meanwhile, melt the chocolate chips in a double boiler, then let cool to room temperature. It is very important that the chocolate and sour cream be the same temperature, otherwise the icing will be lumpy or grainy. (Test it by stirring a little of the sour cream and chocolate together in a bowl; if it mixes smoothly, it's ready.) Stir in the sour cream, 1/ 4 cup at a time, until the mixture is smooth. Taste some! It's good.
5. When the cake is cool, you may frost it as is or cut it in half so that you have two layers (when I do this, I use 2 cups chocolate chips and 2 cups sour cream). My mother uses any leftover icing to make flowers on top. She dabs small rosettes, or buttons, on top, then uses toasted almond slices as the petals, pushing them in around the base of the rosette. (My mother kept it in the fridge, and it is sublime even when cold.)
Recipe from Judith Hesser as heard on NPR.