Butter a 9x13-inch shallow baking dish (with handles, if possible) with 2 tsp. of the butter. Spread the rest of the butter on one side of each slice of the dried bread.
Pour the milk and cream into a medium saucepan and stir in 3/4 cup of the sugar and the salt. Heat on medium high, stirring once or twice, until the sugar is dissolved and the mixture is hot. Meanwhile, melt the chocolate in a double boiler set over—but not touching—hot water. In a large bowl, whisk the eggs and yolks. When the chocolate is melted, stir briefly until smooth and keep it warm.
When the milk is ready, stir in the vanilla and brandy, Grand Marnier, or other liqueur, if using. Gradually whisk about 3/4 cup of the hot milk into the eggs and then whisk in the rest. Measure out 1 cup of the custard, add it to the melted chocolate all at once, and whisk until smooth and glossy (keep the chocolate on the double boiler; it will tighten at first and then get smooth).
Arrange half the bread slices in the buttered pan, buttered side up and overlapping slightly if necessary. Drizzle the bread with half the chocolate custard. Repeat this layering once more. (If you’re using a smaller baking pan, you’ll need to make three layers of bread and chocolate.) Pour the remaining vanilla custard over the bread and press the slices into the custard; the chocolate will smear. Lay plastic wrap directly on the pudding and let sit, pressing on the plastic frequently to submerge the top layer, until the bread is thoroughly soaked, 25 to 30 minutes.
While the bread soaks, adjust an oven rack to the center position. Set a roasting pan, large enough to comfortably hold the pudding, on the rack. Pour in hot water to come one-quarter of the way up the sides. Heat the oven to 350°F.
Remove the plastic and sprinkle on the remaining 2-1/2 Tbs. sugar. Set the pudding in the roasting pan; if the hot water doesn’t come halfway up the sides, add more until it does. Bake until the pudding is slightly puffy and crusty and a wooden skewer inserted in the center comes out clean, 40 to 45 min. Remove the pudding from the water bath. Serve warm.