How to Make It
Preheat the oven to 350°. In a large saucepan, whisk 1 cup of the porter with the molasses and bring just to a boil. Remove from the heat, stir in the baking soda and let cool completely.
In a medium saucepan, whisk the maple syrup with the remaining 1/2 cup of porter and bring to a boil. Reduce the heat and simmer, stirring occasionally, until reduced by half and syrupy, about 20 minutes. Let cool completely.
Meanwhile, butter a 10-inch Bundt pan and dust with flour; shake off the excess. In a large bowl, whisk the 3 cups of flour with the baking powder, cinnamon, nutmeg, salt and cocoa nibs. In a stand mixer fitted with the whisk, beat the 1/2 sticks of butter with the brown and granulated sugars at medium speed until fluffy. Beat in the eggs, 1 at a time, until just incorporated. At low speed, beat in the cooled porter-molasses mixture, then gradually beat in the dry ingredients.
Scrape the batter into the prepared pan and smooth the surface. Bake in the center of the oven for about 50 minutes, until a toothpick inserted in the center of the cake comes out clean. Let cool for 15 minutes. Invert the warm cake onto a rack and brush with the cooled porter-syrup mixture.
In a medium saucepan, combine the sugar with the water and bring to a boil. Cook over moderately high heat, without stirring, until an amber caramel forms. Remove from the heat and whisk in the cream and whiskey. Cook over low heat for 1 minute. Let cool slightly.
Cut the cake into wedges and serve, passing the warm whiskey caramel at the table.