Lightly grease a 3-quart rectangular baking dish; set aside. Lightly spread one side of toasted bread slices with the 1/4 cup softened butter; set aside.
In a medium saucepan, combine milk, whipping cream, the 3/4 cup sugar, and the salt. Bring mixture to simmering, whisking constantly to dissolve sugar.
In a medium bowl, whisk together the 8 eggs and the vanilla. Gradually whisk about 2 cups of the hot milk mixture into egg mixture. Return egg-milk mixture to saucepan. Gradually whisk about 1 cup of the hot milk mixture into melted chocolate until smooth and glossy. (Mixture may begin to clump and harden, but will become smooth.)
Arrange half of the toasted bread slices, buttered sides up, in the prepared baking dish, tearing slices if necessary to fit. Drizzle with about two-thirds of the chocolate mixture. Top with the remaining bread slices and drizzle with the remaining chocolate mixture. Pour the milk mixture over all; press lightly with the back of a large spoon to moisten bread. Cover with plastic wrap; let stand for 30 minutes, pressing bread frequently so it continues soaking up milk mixture.
Preheat oven to 325 degrees F. Bake, uncovered, about 40 minutes or until a knife inserted in center comes out clean. Cool on a wire rack for 30 minutes.
For sauce, in a small saucepan, melt the 1/2 cup butter. Stir in the 1 cup sugar, the water, and egg yolks. Cook and stir over medium-low heat for 5 to 6 minutes or until sugar is dissolved and mixture is boiling. Remove from heat; stir in bourbon.
If desired, top each serving of warm bread pudding with sweetened whipped cream. Serve with sauce. Makes 12 to 16 servings.