Preheat your oven to 350°F (175°C). Lightly grease an 8-inch square baking pan with cooking spray, then line it with parchment paper. Set aside.
Make the brownie batter: In a medium bowl, whisk together the hot melted butter, oil, and sugar until well combined—about 1 minute. Add the eggs and vanilla extract and whisk again until the mixture becomes lighter in color, about 1 more minute.
Add dry ingredients: Sift in the flour, cocoa powder, and salt. Gently fold the dry ingredients into the wet mixture until just combined. Do not overmix—this keeps the brownies fudgy.
Bake the brownies: Pour the batter into the prepared pan and smooth the top evenly. Bake for 20–25 minutes, or until the center is just set and no longer jiggles. If using a toothpick, it should come out with a few moist crumbs for a fudgy texture.
Soak with espresso: Once out of the oven, let the brownies sit for 2–3 minutes. Then poke holes across the surface and pour the espresso evenly over the top. Allow to cool completely to room temperature.
Make the Mascarpone Cream: In a mixing bowl, whisk together mascarpone cheese and heavy cream until smooth, about 30 seconds.
Add the espresso powder, sugar, salt, and vanilla. Continue whisking for 2–3 minutes until stiff peaks form.
Spread the mascarpone cream evenly over the cooled brownies and smooth the top with a spatula. Dust the top with cocoa powder using a fine sieve.
Carefully lift the brownies out of the pan, peel away the parchment paper, and cut into equal squares.