Brew the espresso.
Leave one batch of the brownies in the baking pan and carefully remove the other, cutting them into 16 equal squares.
Working first with the batch of brownies that are still in the pan, carefully slice into 12 equal squares, then poke the top of them with a fork so that the espresso will absorb into them.
Brush the tops of the brownies in the pan with a generous amount of the espresso.
Next, take half of the whipped mascarpone filling and evenly spread it over the bottom layer of the brownies.
One by one, take the second batch of brownies and dunk them into the espresso, covering all the sides, then place them on top of the whipped mascarpone layer.
Once the brownies are laid out and the layer is finished, top the brownies with half of the mascarpone filling that remains. Spread into an even layer.
Add the remaining mascarpone topping into a plastic bag with the tip cut off or a piping bag and pipe dollops of the topping all over the top. Don't worry if it doesn't look perfect, it doesn't have to! It's just supposed to be fun!
Finally, generously dust the entire top with cocoa powder.
Serve right away or out of the fridge.