Preheat the oven to 350°F (175°C). Line a baking pan with parchment paper, leaving some overhang for easy removal.

Melt the butter and bittersweet chocolate chips together in a heatproof bowl over a bain-marie, stirring until smooth. Set aside to cool slightly.

In a separate bowl, whisk the eggs, sugar, brown sugar, vanilla extract, and salt until smooth and well combined.

Gradually pour the slightly cooled chocolate mixture into the egg mixture, stirring continuously until incorporated.

Sift the cocoa powder and flour into the bowl. Fold gently until no dry streaks remain. Avoid overmixing.

Pour the brownie batter into the prepared pan and smooth the top. Bake for about 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Let the brownies cool completely to room temperature.

While the brownies cool, prepare the mascarpone cream. Add the mascarpone, heavy cream, sugar, salt, vanilla extract, and instant coffee powder to a mixing bowl.

Using a hand mixer or stand mixer, whip the mixture until fluffy, smooth, and well incorporated.

Spread the mascarpone cream evenly over the completely cooled brownies.

Refrigerate the assembled tiramisu brownies for at least 2 hours, preferably overnight, so the topping can set and the flavors can meld.

Just before serving, dust generously with cocoa powder. Slice and serve chilled.