Preheat oven to 325°F (160°C) and line a baking sheet with parchment paper.

Beat egg whites and cream of tartar in a clean bowl until stiff peaks form.

Whisk egg yolks, 1/4 cup erythritol, and 1 tsp vanilla in a separate bowl until pale and thick.

Gently fold the almond flour and salt into the yolk mixture, then carefully fold in the egg whites in three batches.

Pipe the batter into 3-inch strips on the baking sheet and bake for 15 minutes until firm but not browned; let cool completely.

In a chilled bowl, whip heavy cream until stiff peaks form.

In a separate large bowl, beat mascarpone, powdered monk fruit, and remaining 1 tsp vanilla until smooth and creamy.

Gently fold the whipped cream into the mascarpone mixture until fully incorporated.

Combine the chilled espresso and rum in a shallow bowl.

Briefly dip each ladyfinger into the coffee mixture (1-2 seconds per side) and layer half of them in the bottom of an 8x8 inch dish.

Spread half of the mascarpone cream over the ladyfingers, then repeat with the remaining dipped ladyfingers and cream.

Dust the top layer with cocoa powder and refrigerate for at least 4 hours to allow the flavors to meld and the structure to set.