Set the oven to 350°F, butter an 8x8-inch pan, and fit parchment across its bottom and sides with excess extending above the rim. The overhanging paper will provide handles for lifting the finished dessert.

Heat the butter in a medium pot over medium heat until it becomes liquid and begins to sizzle. Stop at the first sizzling sounds rather than letting the butter brown.

Move the pot off the burner and whisk the chopped chocolate and espresso powder into the hot butter immediately. Continue until the chocolate disappears into a smooth mixture with a roasted coffee aroma.

Transfer the chocolate mixture to a large bowl and whisk in the granulated sugar while the mixture is still hot. A sandy appearance at this stage is expected.

Incorporate the eggs individually with a whisk, then blend in the vanilla, vegetable oil, cocoa powder, and salt. The batter should become glossy and fall from the whisk in thick ribbons.

Fold in the flour, stopping as soon as it blends into the chocolate batter. No dry pockets should remain, but additional stirring can toughen the crumb.

Scrape the batter into the lined pan and level it across the corners. An even depth helps the center and edges bake more consistently.

Bake on the center rack for 27 to 32 minutes, checking the middle with a skewer. The top should look softly matte rather than wet, and the skewer should carry moist crumbs without liquid batter.

Leave the brownie in its pan until it reaches room temperature throughout. The center must feel completely cool before it can support the biscuit and cream layers.

Stir the boiling water, espresso powder, and granulated sugar together in a wide bowl, then let the mixture reach room temperature. The powder and sugar should dissolve completely before the coffee cools.

Dip each ladyfinger briefly into the cooled coffee for no more than about one second total, then place it over the cooled brownie, trimming biscuits where needed. The biscuits should feel damp on the surface while remaining firm enough to move intact.

Keep the biscuit-covered brownie in the pan while you prepare the topping. The ladyfingers should form an even layer without a pool of coffee around them.

Briefly beat the cold mascarpone, powdered sugar, and vanilla in a large bowl using an electric mixer or a stand mixer with its paddle attachment. Stop once the mixture looks smooth and evenly blended.

Run the mixer on low and feed in the cold cream slowly, scraping the bowl after each 1/4 cup; alternatively, incorporate 1 to 2 tablespoons at a time on low. Each addition should blend smoothly before more cream enters the bowl.

Increase the speed to high after all the cream is incorporated and whip for about 1 to 2 minutes. Stop as soon as the topping forms billowy stiff peaks that stay upright.

Distribute spoonfuls of topping across the ladyfingers and gently connect them into an even layer. Light spreading keeps the biscuits in place and preserves the cream’s airy texture.

Refrigerate the assembled dessert for at least 1 hour, continuing until the topping feels firm. The cream should hold its shape rather than shift when the pan moves.

Lift the dessert onto a cutting board using the parchment handles, sift cocoa over the cream, and cut into bars. The chilled layers should remain distinct as the knife passes through them.