Preheat oven to 350°F.

Grease and flour sides of a 9”x13”baking pan; line bottom of pan with parchment paper. Set pan aside.

Using a medium mixing bowl, sift together the flour, baking soda and salt; set aside.

In a separate bowl, whisk together the cocoa powder and coffee until well combined. Stir in the milk and vanilla extract; set aside.

Using a countertop mixer fitted with the paddle attachment, cream the butter and both sugars together on medium-high speed until light and fluffy (~4-5 minutes).

Add the eggs one at a time, mixing until just incorporated after each addition.

Add half of the flour mixture to the bowl; mix on low speed until just combined.

Add half of the liquid mixture; mix on low speed until just combined.

Repeat with the remaining flour and liquid.

Pour batter into prepared pan. Bake at 350°F for 36-38 minutes, or until a toothpick inserted into the center of the cake comes out mostly clean.

Let cake cool in the pan for at least 30 minutes, then cover and transfer to a freezer for at least 60 minutes, or overnight.

Using a countertop mixer, cream together the butter and marshmallow fluff until smooth (~2-3 minutes on medium high speed).

Add vanilla and milk and powdered sugar 1 cup at a time, mixing well after each addition.

Place buttercream in refrigerator for 15-20 minutes.

Using a medium heatproof mixing bowl, whisk together powdered sugar, milk and vanilla extract; set bowl aside.

Using a medium saucepan, add sugar, cocoa powder, corn syrup, salt and ½ cup (1 stick) of butter. Place over medium-high heat and bring to a boil, stirring occasionally. Continue cooking until mixture reaches 240°F.

Remove saucepan from heat and slowly pour liquid into bowl with the powdered sugar, whisking constantly as you pour. Add remaining ½ cup (1 stick) of butter and stir until butter has completely melted.

Let mixture cool until just warm to the touch.

Remove the chocolate cake from the freezer and slice into 3”x3” squares.

Using two spoons (or a large piping tip), place a large scoop of buttercream onto the center of each square. Place squares back in freezer for 30 minutes.

Remove squares from freezer and pour warm (not hot) frosting evenly on top. (Tip: Place squares on a cooling rack set over a baking pan. That way, the pan catches any excess frosting as it’s poured.)

Place squares back in freezer for 15 minutes, or until chocolate frosting has hardened.

Serve chilled and store any leftovers in the refrigerator.