1. Heat oven to 350F. Lightly grease an 8 or 9 inch square baking pan.
2. Whisk together brown sugar, flour, cocoa powder, baking soda, and salt in a medium bowl. Whisk together egg, buttermilk, oil, and vanilla in a small bowl. Add buttermilk mixture to flour mixture, whisking to combine. Gradually whisk in hot coffee.
3. Pour batter into prepared pan and bake 25-30 minutes or until a wooden pick inserted in the center comes out clean. Cool completely on a wire rack, about 1 hour.
4. To make frosting, combine fudge, cream, and salt in a medium saucepan over medium heat, stirring until melted and smooth, about 5 minutes. Immediately pour over cake. Let stand 15 minutes before serving to allow the icing to set.
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