Place chocolate and 1 cup plus 1 1/2 tablespoons butter in top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until melted, about 5 minutes.

Beat eggs, vanilla, and salt together in a bowl using an electric mixer until creamy. Add sugar; blend well. Stir in chocolate-butter mixture with a spoon. Add flour, about 2 tablespoons at a time, mixing well after each addition until fondant mixture is smooth.

Butter a 9-inch round pan. Pour the fondant mixture evenly into the pan; cover with aluminum foil. Freeze until firm, 4 hours to overnight.

Preheat oven to 356 degrees F (180 degrees C).

Bake the fondant in the preheated oven until sides are firm and center is still liquid, about 40 minutes.