Preheat your oven to 350°F (180°C) and grease a cake pan.
Cream together the butter and erythritol until light and fluffy. Add the eggs one at a time, mixing well after each addition.
In a separate bowl, whisk together the flour and baking powder. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour the batter into the prepared cake pan and bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely in the pan before removing.
For the cream, combine the mascarpone cheese, heavy cream, and vanilla extract in a bowl. Beat until smooth and fluffy.
Slice the cooled cake in half horizontally. Moisten the bottom layer with half of the coffee. Spread half of the mascarpone cream over the moistened cake.
Place the second layer on top, moisten with the remaining coffee, and frost the entire cake with the remaining mascarpone cream.
Dust the top with cocoa powder. Refrigerate the cake for at least 2 hours before serving to allow the flavors to meld.
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