In a medium bowl, soak the cranberries in the rum for at least 2 hours or overnight.
Drain the cranberries and reserve them and the soaking liquid separately. Set aside 2 tablespoons of the rum to flavor the whipped cream. Preheat the oven to 325 degrees . Grease two 9-inch round cake pans (or springform pans) with cooking spray, then cover the bottom of each with a circle of parchment paper and spray again.
In a medium saucepan, stir the butter and chocolate together over low heat until melted. Remove the mixture from the heat and scrape it into a bowl. Whisk in 2 cups of the sugar and the salt until well combined. Whisk in the eggs, 1 at a time, then sift in half of the flour, 1/3 cup of the cocoa and the baking powder and stir until no floury streaks remain. Repeat with the remaining flour and 1/3 cup of cocoa. Fold in the cranberries. Pour the batter into the prepared pans and bake for 45 minutes, or until a toothpick inserted in the center of a cake layer comes out clean.
Cool the cakes on a rack for 10 minutes. Loosen the cakes from the sides of the pans with a knife and invert, 1 at a time, onto a plate and remove the parchment paper. Turn the cakes right side up and return to the rack. Prick the tops of the cakes with a toothpick in several places and drizzle with the reserved rum. Let the cake layers cool completely. (They will stay fresh for a couple of days when covered with plastic wrap and stored at room temperature.)
Up to 3 hours before serving, pour the heavy cream into a deep bowl, add the remaining 2 tablespoons of sugar and 2 tablespoons of reserved rum and whip with an electric beater until thick and billowing. Do not whip the cream to stiffness or it will separate.
Place 1 cake layer on a serving plate and spoon a 1/2 inch of the whipped cream on top, then add the second layer and pile on the rest of the cream. Sift the remaining 1 teaspoon of cocoa powder over the top.
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