Preheat your oven to 350°F and prepare a 9x9-inch cake pan with parchment paper and butter/oil.

If making your own buttermilk, mix 2/3 cup milk with 2 tsp vinegar or lemon juice, stir, and set aside.

In a bowl, mix the flour, sugar, baking powder, baking soda, and salt. Sift to remove any lumps.

Add the egg, vegetable oil, and buttermilk. Whisk until combined.

In a smaller bowl, mix the boiling water with cocoa powder and instant coffee until smooth and shiny.

Add the cocoa mixture to the batter and gently fold until combined.

Pour the batter into the prepared pan.

Bake for about 23-25 minutes, until a toothpick inserted into the center comes out with moist crumbs.

Let the cake cool in the pan for at least 20 minutes.

Whisk the chilled heavy cream, sugar, and vanilla in a bowl until it thickens (do not overmix).

Add the mascarpone cheese in two additions and mix on low speed until combined. The mixture should be thick and fluffy. Avoid overmixing to prevent graininess.

Once the cake has cooled, carefully remove it from the pan and trim off the crust.

Clean the pan and line it with two sheets of parchment paper, leaving the edges overhanging.

Transfer the cake back into the pan, with the trimmed side up.

Use a pastry brush to generously brush the strong coffee or espresso over the cake.

Spread the mascarpone whipped cream evenly over the cake.

Refrigerate for about 6 hours, or ideally overnight, to set.

Once set, carefully lift the cake out of the pan using the parchment paper edges and place it on a serving board.

Dust the top with cocoa powder using a sifter.

Slice, serve, and enjoy!