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Preheat the oven to 350°F. Grease a 13 x 9 inch baking pan (with butter, oil, or cooking spray).

To make the cake: In a medium bowl, whisk together the boiling water and cocoa. Stir in the melted butter. Whisk in the eggs, buttermilk, and vanilla. In another medium bowl, whisk together the flour, both sugars, the baking soda, salt and baking powder. Fold the flour mixture into the chocolate mixture until combined. Pour the batter into the prepared pan. Bake until a toothpick inserted n the center of the cake comes out clean, 30 to 35 minutes.

While the cake is baking, prepare the frosting: In a large saucepan, whisk together the sugar and cocoa. Whisk in the milk. Then stir in the butter and corn syrup. Place the pan over medium-high heat and bring to a boil, whisking constantly. Off the heat, stir in the confectioners’ sugar and vanilla.

Spread the frosting over the surface of the cake just as soon as it comes out of the oven. Let the cake cool on a rack, then serve it up.