1. To make the dark chocolate mousse, melt the 6 ounces chocolate and the butter in the top of a double boiler over, but not touching, barely simmering water. Heat and stir until the chocolate is melted and smooth. Set aside until it is near room temperature but still fluid.

2. Whip 3 egg yolks with 2 tablespoons of the sugar in a standing mixer using the whip attachment at medium speed until the mixture is pale and light, about 5 minutes. Fold in the cooled chocolate.

3. Whip ¼ cup of the heavy cream with the 1 tablespoon Kahlua to medium peaks. Gently fold into the chocolate mixture.

4. In a large, clean, dry bowl, whip 2 egg whites with 2 tablespoons of the sugar until they just begin to hold a stiff peak but are not at all dry. Gently fold into the chocolate mixture. Cover with plastic wrap and refrigerate until thickened and spreadable, at least 1 hour.

5. To make the mascarpone cream, whisk together the remaining ½ cup cream, ½ cup of the sugar, and the vanilla in a medium bowl until the sugar dissolves. Add the mascarpone, whisking until the mixture is smooth and thick. Set aside. (Refrigerate if not using immediately.)

6. To make the coffee syrup, stir together the coffee and brandy with the remaining ¼ cup sugar and the ¼ cup Kahlua until the sugar is completely dissolved.

7. To assemble the tiramisu, put 12 ladyfingers into a small (10 by 7-inch) glass or ceramic baking dish or serving dish, cutting them as needed to (mostly) cover the bottom of the dish. Use a pastry brush to soak the ladyfingers with coffee syrup, using about one third of the syrup and painting them a couple of times as the syrup absorbs into the cookies.

8. Dollop and spread half of the mascarpone cream evenly over the ladyfingers. Top with another 12 ladyfingers and brush them generously with the syrup.

9. Dollop and spread all of the dark chocolate mousse evenly over the ladyfingers. Top with the final 12 ladyfingers and brush them with the remaining syrup.

10. Use a grater or sharp knife to shave about 1 ounce chocolate into small pieces. Fold the chocolate into the remaining mascarpone cream, then dollop and spread the mixture evenly over the ladyfingers to form the top layer.

11. Lightly press a piece of waxed paper over the tiramisu, then wrap the dish with plastic wrap and refrigerate for at least 2 hours or up to 2 days.

12. To serve, cut the tiramisu with a sharp knife dipped in warm water and wiped with a clean towel. Carefully lift the pieces onto dessert plates with a small, flexible spatula. Top with additional chocolate shavings or sifted cocoa, if desired.