Spread one side of each slice of bread with the butter and cut the bread into 1/2-inch pieces. In a small saucepan melt the chocolate in the milk over moderate heat, whisking until the mixture is smooth. In a bowl whisk the egg with the sugar, add the hot mixture in a stream, whisking, and stir in the vanilla and the bread. Transfer the mixture to a buttered loaf pan, 9 by 5 by 3 inches, let it stand for 5 minutes, and bake it in the middle of a preheated 375°F. oven for 25 to 35 minutes, or until a knife inserted in the center comes out clean. Serve the bread pudding warm and the ice cream.
