Heaven and Hell Sheet Cake

1/2 cup unsalted butter, room temperature
2 cups packed light brown sugar
2 large eggs, room temperature
For the frosting:
3/4 cup plus 2 tablespoons sugar
1 tablespoon corn syrup
4 egg whites
1 1/4 teaspoons vanilla extract
To make the cake:
Preheat oven to 350°F and grease a 9×13 baking pan with baking spray, set aside.
In a bowl whisk flour, baking soda, and salt, set aside.
In another bowl combine hot coffee, cocoa powder, and chopped chocolate, whisking until chocolate is dissolved. Let the mixture cool slightly. Once slightly cooled down, whisk buttermilk with coffee-cocoa mixture. Set aside.
In a different bowl, beat oil, butter, and brown sugar until completely combined. Add eggs one at a time beating well between each addition.
Gradually add the dry ingredients in 3 additions alternately with the wet ingredients in 2 additions, beginning and ending with the dry-ingredients. Use a rubber spatula to scrape the sides and bottom of the bowl, ensuring proper incorporation.
Pour batter into prepared pan and bake until a toothpick inserted comes out mostly clean with a few moist crumbs, about 25 to 30 minutes.
Cool completely.
To make the frosting:
Fill a small saucepan with water and set over medium-low heat, let it get to a simmer and then reduce the heat.
In a mixing bowl combine sugar, corn syrup, and egg whites.
Place mixing bowl over the pot of simmering water and constantly whisk for 7 minutes. The mixture will look opaque and smooth.
Transfer to a mixing bowl, and with a whisk attachment, whip until the mixture doubles in size and has a fluffy, marshmallow consistency. Add vanilla, whipping to combine.
Frost onto cooled cake and serve.