Place Guinness and molasses in a saucepan and bring to a boil, whisking to combine. Remove from the heat and add the baking soda. Set aside to cool down.
Preheat the oven to 350ºF. Grease the cake pans with butter. Line the bottoms with parchemnt paper and grease again.
In a mixing bowl whisk the flour, the spices, baking powder and salt.
In a different bowl mix the eggs, the granulated and dark brown sugars and the oil until smooth. Slowly stir in the beer mixture.
Whisk in the wet mixture into the dry mixture. To form a smooth batter. Divide the batter evenly between the two cake pans. Bake for about 30 minutes, but check at the 25 minute mark to not to overcook (I like to remove the cakes from the oven when a toothpick inserted in the center of the cakes comes out “almost’ dry but not completely, since the cake will continue baking outside of the oven).
Make the icing:
Place the cream cheese, butter, rum or rum extract and nutmeg in a stand mixer fitted with the paddle. Beat at medium speed until smooth, about 1 minute. Reduce the speed and slowly add in the icing sugar (do it in 3 or 4 additions and mix well after each addition).
Using a serrated knife, split each cake in half horizontally. You’ll end up with 4 layers. Place one layer on a cake stand or plate. Spread about 1/4 of the icing and top with a second layer. Add 1/4 cup cream cheese frosting and top with the third cake layer. Spread another 1/4 of the icing. Top with the last cake layer, placed inverted so the flattest side is up. Spread the remaining cream cheese frosting on top and some on the sides. Smooth the sides with a flat wide spatula. Place the iced cake in the fridge for 1 hour to set the icing.
If making ahead off time, cover with plastic and keep in the fridge for up to 3 days.
Make the sugared cranberries:
Place 1/2 cup sugar and 1/2 cup water in a small saucepan and simmer until the sugar dissolves. Add the cranberries and cook until the berries start to soften, 1 minute. Using a slotted spoon, transfer the cranberries to a rack set over a plate. Let cool until no longer sticky, about 30 minutes.
Place 1/3 cup sugar and 1 tsp finely grated lime zest in a small bowl and stir to mix. Toss the sugared cranberries in the lime sugar until covered. Pile the sugared cranberries on top of the cake before serving
Notes
The leftover lime sugar can be stored in an airtight container for many weeks