To make the almond flour cake:
Preheat oven to 325°F (165°C). Line an 8x8-inch baking pan with parchment paper and lightly grease the sides.
Whisk almond flour, baking powder, and salt in a small bowl.
Separate the eggs. In a large bowl, beat egg whites with cream of tartar until soft peaks form. Gradually add 1 tbsp erythritol and beat to stiff peaks.
In another bowl, beat yolks with remaining erythritol, stevia, vanilla, and almond extract until thick and pale.
Gently fold yolk mixture into egg whites. Then carefully fold in the almond flour mixture.
Spread batter in the prepared pan and bake 18–20 minutes until golden and springy. Cool completely and cut into 16 ladyfinger-sized strips.
To make the mascarpone custard:
In a heatproof bowl, whisk yolks, erythritol, and stevia until pale.
Place over a pot of simmering water (double boiler) and cook for about 10 minutes, stirring constantly, until slightly thickened.
Remove from heat and beat in mascarpone until smooth.
In a separate bowl, whip cold heavy cream to stiff peaks. Gently fold whipped cream into the mascarpone custard with vanilla and coffee extracts (if using).
To assemble:
Mix brewed coffee and rum in a shallow dish. Lightly dust the bottom of an 8x4-inch loaf pan with cocoa powder.
Lightly dip each almond cake strip into the coffee mixture and layer in the pan.
Spread half the mascarpone custard over the first cake layer. Dust with cocoa powder.
Repeat with remaining dipped cake strips and custard. Finish with cocoa powder on top.
Cover and refrigerate for at least 8 hours (overnight preferred) before serving.