Spread the croissants in a large baking pan. Dry, uncovered, at room temperature for 12 hours or heat in the oven at 250 degrees for 1 hour.
Butter a shallow V/2-quart baking dish. Arrange half the croissants in a single layer in the baking dish. Sprinkle half the chocolate evenly over the croissants. Top with the remaining croissants.
Whisk the eggs, sugar, milk, 1/2 cup of the cream, vanilla and 1/4 teaspoon salt in a bowl; pour over the croissants. Cut the butter into pieces; dot over the croissant mixture. Chill, covered, for 1 to 24 hours.
Preheat the oven to 350 degrees. Bake the pudding on the middle rack for 40 minutes or until slightly puffed and golden. Melt the remaining chocolate in the top of a double boiler over barely simmering water. Whisk in the remaining 1/4 cup cream and a pinch of salt. Cook until well mixed, whisking constantly. Keep warm, covered.
Serve the pudding warm or at room temperature with the sauce.