Line a 12 1/2 x 4 1/2inch sharp-edged loaf pan with enough foil to hang over the sides about 3 inches. Chop the chocolate into small pieces and melt with the butter in the top of a double boiler or in a bowl over simmering water. Remove from the heat and stir in the Kahlua. Whisk in the egg yolks until well combined.
Whisk the egg whites until soft peaks form. Gently fold the whites into the chocolate mixture in two stages.
Pour into the prepared pan, tap on the counter to remove air, smooth top, and cover with plastic wrap touching the top surface. Chill 6 hours or overnight.
To serve, remove the plastic wrap and invert onto a serving platter. The chocolate should release easily. Remove the foil.
Coat dessert plates with Espresso Creme Anglaise. Top each with a slice of chocolate. (This slices most easily with a long, thin knife that has been dipped into hot water.)
In a large bowl, whisk together the egg yolks and sugar.
Line a sieve with a paper coffee filter.
Combine the milk and coffee in a medium saucepan and bring to a boil. Pour the hot milk and coffee through the sieve and into the egg mixture, whisking constantly. Return to the saucepan.
Cook over a medium heat, stirring constantly, until the mixture thickens slightly and coats the back of a wooden spoon, about 5 minutes. Remove from the heat. Stir in the vanilla and chill.
c.1997, M.S. Milliken & S. Feniger, all rights reserved