CAKE
Preheat the oven to 350 degrees F.
Add the dry ingredients: flour white and brown sugar, baking soda, baking powder, cocoa powder and salt to a large bowl, combine well.
Whisk together the wet ingredients - the milk, eggs, melted butter, and vanilla.
In a stand mixer or using a hand mixer - mix the batter until well combined, but don't overdo it!
Add boiling water to the mix, and carefully blend.
Add to a greased 9x13 cake pan and bake in the oven for 25-30 minutes (a toothpick should come out clean when inserted in the center).
Remove from the oven, and cool in the pan on a wire rack for 5 minutes, then carefully remove and cool completely.
Frost as desired.
FROSTING
Melt the chocolate in a microwave-safe bowl, in 30-second increments at 60% power, stirring until smooth.
Add the powdered sugar, coffee powder, salt, and butter to a large bowl and gently combine.
Add the milk, vanilla, and melted chocolate.
Whip until smooth.
Frost the cooled cake.