Combine the coffee and Kahlua and allow to cool before assembling the tiramisu.

Beat the egg yolks with 4 tablespoons of sugar until very pale yellow.

In a separate bowl, beat the egg whites and cream of tartar until soft peaks form.

Beat the mascarpone into the egg yolk mixture until smooth.

Gently fold the egg whites into the egg yolk and mascarpone mixture.

In a 9x9 inch pan, spread a thin layer of the cream mixture on the bottom.

Soak the ladyfingers in the coffee mixture briefly and layer them over the cream.

Spread half of the remaining cream over the ladyfingers.

Add another layer of soaked ladyfingers and top with the rest of the cream.

Dust the top with unsweetened cocoa.

Refrigerate overnight or for at least 4 hours before serving.