Preheat oven to 350 F. Prep your cupcake tins by lining with cupcake liners and spraying with cooking spray. Either baking spray or regular will work. Prep your 8" cake pan by spraying with cooking spray, lining the bottom with a circle of parchment paper, and then spraying again.
For the cake: Into a medium sized bowl sift together the flour, salt, baking powder, baking soda, cocoa, and sugar. Set aside. In the large bowl of a stand mixer, combine oil, coffee, and milk. Then slowly mix in the dry ingredients until fully combined. Then add eggs and vanilla and beat on medium speed for 3-5 minutes.
Fill liners 2/3 of the way full. Then put the remaining batter into the cake pan. The batter will be very thin. Bake at 350 F for 15-20 minutes for the cupcakes, or until a toothpick comes out clean. The cake will require an additional 5 minutes or so. Remove from oven and let cool for 5 minutes in the pans, and then remove cupcakes from the cupcake tins and let cool completely. Remove the cake from the pan as well by inverting onto a wire cooling rack.
Make the filling while your cakes cool by creaming the butter and adding 1/2 cup of the powdered sugar. Add vanilla and 1 TBSP of cream. Beat until well combined, scraping down the sides as necessary. Add last 1/2 cup of powdered sugar and remaining 2 TBSP of heavy cream alternating between the two and mixing well between each. Beat in the marshmallow cream last.
Place the filling in a piping bag with a filling tip, and poke into center of completely cooled cupcakes, moving up and down as you gently squeeze approximately 1/2 – 1 TBSP of filling into each cupcake. If you are making a cake as well, cut the cake in half after it is completely cooled, and pipe filling onto the middle of the cake and reassemble.
To make the ganache, heat your 1/2 cup of heavy cream and 1 TBSP butter until almost boiling. Pour over chocolate chips in a metal bowl and stir well until the chocolate is completely melted. Add the 2 tsp vanilla and stir to combine.
Dip the filled cupcakes into the ganache to coat. Pour the remaining ganache over the top of the cake and spread evenly, letting a little bit spill down the sides. Place the cupcakes/cakes in the refrigerator until the ganache is set, about 30 minutes.
To make the icing, beat the 1 stick of softened butter, 2 TBSP milk, 2 cups powdered sugar, and 1/4 tsp vanilla until smooth and fully combined. Place into a piping bag (or a baggie!) with a small tip attached, and pipe loops onto each upcake after the ganache is fully set.