Preheat the oven to 375°F and grease a 15x10-inch jelly roll pan with non-stick cooking spray and set aside.

In a large pot over medium heat, melt the butter and whisk in the cocoa.

Add the oil and coffee and bring to a rolling boil for 30 seconds. Remove from the heat and set aside.

Sift together the flour, sugar, baking soda, and salt and add it to the butter mixture mixing just until combined.

In a separate bowl, beat together the eggs, buttermilk, and vanilla before adding it to the rest of the ingredients and mixing it with a rubber spatula until fully incorporated.

Pour the batter into the baking pan and bake for 20 to 25 minutes, or until a toothpick comes back mostly clean from the center of the cake.

While the cake is baking, prepare the frosting by melting the butter in a medium saucepan over medium heat.

Add the cocoa and bring the mixture to a rolling boil for 30 seconds.

Remove from the heat and whisk in the milk and vanilla.

Add the powdered sugar while whisking constantly until smooth.

As soon as the cake comes out of the oven, pour the frosting over the top of the warm cake and top with chopped hazelnuts, if desired.

Allow the cake to cool completely before slicing and serving.