Preheat your oven to 350F and line a 9x9-inch baking pan with parchment paper. Melt the butter and sugar-free dark chocolate together in a microwave-safe bowl in 20-second intervals until smooth. Whisk in the granulated erythritol, then beat in the eggs one at a time. Stir in the 2 tbsp cooled espresso.
Fold in the almond flour, cocoa powder, baking powder, and salt until a thick batter forms. Spread evenly into the prepared pan. Bake for 20 to 22 minutes until set but still fudgy. Let the brownie base cool completely in the pan.
Mix the cooled espresso with 1 tbsp powdered erythritol for the coffee soak. Once the brownies are completely cool, brush the coffee soak evenly over the entire surface using a pastry brush. Let it absorb for 5 minutes.
Beat the mascarpone cheese and powdered erythritol together until smooth. In a separate chilled bowl, whip the heavy cream and vanilla extract to stiff peaks.
Gently fold the whipped cream into the mascarpone mixture in two additions until smooth and fluffy.
Spread the mascarpone cream layer evenly over the coffee-soaked brownie base. Smooth the top with an offset spatula.
Refrigerate for at least 4 hours or overnight until the cream layer is firm and the flavors meld together.
Before serving, dust the top generously with unsweetened cocoa powder through a fine mesh sieve. Slice into 12 bars and serve cold.
