Place a rack in the center of your oven and preheat it to 350°F. Grease a jelly roll pan (10x15) with a 2-inch rim with melted butter. Lightly flour the pan. Shake off any excess and place in the freezer.
In a large bowl, combine the flour, granulated sugar, brown sugar, salt, baking powder, and baking soda.
In a saucepan over medium heat, melt the butter, then brown it. Take the pan off the heat.
Add the unsweetened cocoa powder to the brown butter, and stir to incorporate.
Place the pan back over medium heat. Add the hot coffee, and stir. Take the pan off the heat.
Add the eggs and vanilla to the mayonnaise and combine. Add a few tablespoons of chocolate to temper (warm) the egg mixture.
Once the egg mixture is warm, mix the chocolate and egg mixture together.
Pour the wet ingredients into the dry ingredients and stir just enough to combine.
Add the batter to the sheet pan and bake for 15 to 20 minutes, until the cake has a slight spring when you press a finger on its surface and a toothpick inserted into the middle comes out clean.
Once the cake is out of the oven, make the icing. Set a large saucepan over medium heat, and add the butter, heavy cream, cocoa, vanilla, and powdered sugar. Stir until incorporated.
Pour 3/4 of the warm frosting over the center of the cooled cake. Begin to spread it over the cake with a spatula and add more frosting if necessary to cover the cake from end to end. Cut, serve, then sit back and enjoy the marathon of compliments you're going to get!