Position a rack in the center of the oven and heat the oven to 350 degrees F. Butter a 10- or 12-cup bundt pan (or twelve 1-cup mini bundt pans) and then lightly coat with sifted cocoa powder. Tap out any excess cocoa.

In a small saucepan over high heat, bring the stout and molasses to a simmer. Remove the pan from the heat and let stand while preparing the cake batter.

Sift together the flour, cocoa powder, baking powder, baking soda, and salt. With a stand mixer (use the paddle attachment) or a hand mixer, cream the butter in a large bowl on medium speed until smooth, about 1 minute. Add the brown sugar and beat on medium speed until light and fluffy, about 3 minutes. Stop to scrape the sides of the bowl as needed. Beat in the eggs one at a time, stopping to scrape the bowl after each addition. With the mixer on low speed, alternate adding the flour and stout mixtures, beginning and ending with the flour. Stop the mixer at least one last time to scrape the bowl and then beat at medium speed until the batter is smooth, about 20 seconds. Stir in the chopped chocolate.

Spoon the batter into the prepared pan (or pans), spreading it evenly with a rubber spatula. Run a knife through the batter to eliminate any air pockets. Bake until a wooden skewer inserted in the center comes out with only a few moist crumbs clinging to it, 45 to 50 minutes (about 35 minutes for mini cakes). Set the pan on a rack to cool for 20 minutes. Invert the cake onto the rack and remove the pan. Let cool until just barely warm.

Make the glaze, if using: Bring the cream to a boil in a small saucepan over high heat. Remove the pan from the heat and add the chocolate. Let stand for 1 minute and then whisk until the chocolate is melted and smooth. Let cool for 5 minutes.

Drizzle the barely warm cake with glaze and then let cool to room temperature before serving.

Make Ahead Tips: Wrapped tightly in plastic, the cake keeps for up to a week, or you can freeze it for up to a month. If you're making the cake ahead, wrap it while still barely warm without the glaze. If you plan to freeze the cake, don't glaze it until you're ready to serve it or give it away.