Preheat the oven to 350°F (175°C). Line a 9x9-inch square baking pan with baking paper.
In a large mixing bowl, beat the egg whites with cream of tartar until stiff peaks form.
In another bowl, beat the egg yolks with Erythritol until pale and fluffy. Mix in vanilla extract.
Gently fold the almond flour, coconut flour, and baking powder into the egg yolk mixture until well combined.
Carefully fold the whipped egg whites into the batter until no streaks remain.
Pour the batter into the prepared baking pan and spread it out evenly. Bake for 15-20 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool completely.
Place the cooled coffee in a shallow dish. Set aside.
In a mixing bowl, beat the mascarpone cheese, heavy cream, powdered Erythritol, and vanilla extract until smooth and creamy.
Once the cake has cooled, cut it into lady finger cookie shapes.
Dip each cake piece into the coffee briefly, ensuring they are moistened but not soggy.
Arrange a layer of soaked cake pieces at the bottom of your serving dish.
Spread half of the mascarpone filling over the soaked cake layer.
Repeat with another layer of soaked cake pieces and the remaining mascarpone filling.
Before serving, dust the top with unsweetened cocoa powder. Slice and serve chilled.