Wellesley Fudge Cake

Ingredients

• For the cake:
• 2 1/2 cups flour
• 2 teaspoons baking soda
• 1 teaspoon baking powder
• 1/2 teaspoon salt
• 3/4 cup hot water
• 1/2 cup Dutch-processed cocoa powder
• 1 cup unsalted butter, softened
• 2 cups granulated sugar
• 2 large eggs
• 1 cup buttermilk, at room temperature
• 2 teaspoons vanilla extract
• For the frosting:
• 1 1/2 cups packed light brown sugar
• 1 cup evaporated milk
• 1/2 cup unsalted butter, softened and cut into 1/2 inch pieces
• 8 ounces bittersweet chocolate, coarsely chopped
• 1 teaspoon vanilla extract
• 3 cups confectioners' sugar, sifted

Directions

1. For the cake, set an oven rack in the center position and preheat to 350 degrees F. Prepare 2 square or round 8-inch baking pans by greasing and flouring them, then fit parchment paper into each pan bottom. Whisk the salt into the flour, baking soda, and baking powder in a bowl; set aside.
2. In a small saucepan, warm the cocoa and hot water together until smooth. In a stand mixer bowl fitted with the paddle attachment, beat the butter and granulated sugar on medium-high speed until the mixture is pale and fluffy. Add the eggs 1 at a time, beating until each addition is blended. Add a third of the flour mixture, then mix in 1/2 cup of the buttermilk. Follow with half of the flour mixture that remains and the remaining 1/2 cup buttermilk. Add the last of the flour mixture along with the vanilla extract, mixing only until everything comes together. Drop the speed to low and gradually pour in the cocoa mixture, mixing until it is worked through the batter.
3. Run a rubber spatula through the batter once more to make sure it is evenly mixed. Transfer the batter to the pans you prepared, smooth the tops, and give each pan a light tap against the counter to settle the batter. Bake for 25 to 30 minutes, rotating the pans midway, until a toothpick poked into the center comes back with just a few crumbs clinging to it. Cool the cakes in their pans for 15 minutes. Slide a small knife along the sides of the cakes, then turn the layers onto a wire rack. Remove the parchment, turn the cakes upright, and cool completely before frosting, about 2 hours. If pausing here, wrap the cooled cakes tightly in plastic wrap; they can stay at room temperature for up to 2 days.
4. For the frosting, stir the brown sugar, 1/2 cup evaporated milk, 4 tablespoons butter, and salt together in a large saucepan. Set the pan over medium heat and cook for 4 to 8 minutes, until small bubbles begin to show around the rim. Turn the heat to low and let it simmer, stirring from time to time, until large bubbles rise, the mixture thickens, and the color becomes deep golden brown, about 6 minutes. Pour it into a large bowl. Stir in the remaining 1/2 cup evaporated milk plus the remaining 4 tablespoons butter until the frosting cools slightly. Add the chocolate and vanilla, stirring until smooth. Whisk in the confectioners' sugar until it is fully incorporated. Cool the frosting at room temperature for about 1 hour, giving it an occasional stir.
5. Arrange strips of parchment paper around the outside edge of a cake platter so the platter stays clean while you build the cake. Set a cake layer on the platter and spread 1 cup frosting across it all the way to the edges. Set the second cake layer on top, press lightly so it adheres, then use the rest of the frosting to coat the top and sides evenly. Chill the cake for about 1 hour, until the frosting has firmed. Before serving, pull the parchment strips away from the platter.