Heat the cream in the microwave until steaming and pour over the milk chocolate and salt. Let sit 5 min and then whisk until smooth. Refrigerate until ready to use. When your cake is cool, whip the ganache in a mixer with a whip attachment just until it is fluffy. Don't overwhip or it will separate. Use a rubber spatula or a cake spatula to spread it over the cooled cake. Top with sprinkles if you like.
For the cake:
Adjust over rack to lower middle and heat oven to 350. Place the skillet on the stove over medium heat and add the butter and coffee. Cook until the butter is melted. Remove from the heat and mix in the cocoa and dark chocolate, followed by the brown sugar, vanilla and salt. Whisk until smooth. Add the eggs and whisk, followed by the baking soda and mix well. All the flour and gently combine just until mixed.
Place in oven and bake until the cake is firm, though your finger will still leave a little impression if cake is pressed. A toothpick in the center should have a few crumbs attached. Remove from the oven and cool completely then frost. This stored well both at cool room temp and in the fridge.