For the cake :
1-1/4 cups stout, such as Guinness (don’t include the foam when measuring)
1/3 cup dark molasses (not blackstrap)
7-1/2 oz. (1-2/3 cups) all-purpose flour
2-1/4 oz. (3/4 cup) unsweetened natural cocoa powder (not Dutch-processed); more for the pan
1-1/2 tsp. baking powder
1/2 tsp. baking soda
1/2 tsp. salt
10 oz. (1-1/4 cups) unsalted butter, softened at room temperature; more for the pan
1-1/2 cups packed light brown sugar
3 large eggs, at room temperature
6 oz. semisweet chocolate, very finely chopped

For the glaze: (optional)
3/4 cup heavy cream
6 oz. semisweet chocolate