DARK CHOCOLATE ICING:
12 oz semisweet chocolate, chopped finely
6 tbsp unsalted butter, cut into pieces
1 cup heavy cream
3 tbsp rum (or use 1 tsp rum flavoring)
1 cup cherry preserves
1 semi-sweet (4 oz) chocolate bar
Place 12-ounce chocolate and butter in a large bowl. In a small saucepan, bring cream to a boil over medium heat, stirring occasionally. Pour cream over the chocolate and butter and shake the bowl gently to distribute the cream evenly. Let sit for 2 to 3 minutes. Whisk together by hand until smooth. Cover the surface of the icing with plastic wrap and set aside in a cool spot to set but do not refrigerate. When set, the icing should have the consistency of a firm pudding. Set aside the preserves until the cake is to be assembled.
With a vegetable peeler, shave the chocolate bar into a freezable bowl. Place in the freezer until ready to decorate the cake.
DARK CHOCOLATE CAKE:
1 stick unsalted butter, softened
3/4 cup granulated sugar
1/2 cup light brown sugar, packed
2 large eggs
1 1/2 cups all-purpose flour
1 tsp baking soda
1/2 tsp baking powder
1/4 tsp salt
1/2 cup cocoa powder
1/2 cup sour cream
1/4 cup rum
3/4 cup boiling water
Preheat oven to 350 degrees. Butter and line with parchment paper the bottom of single layer heart-shaped pan. Set aside.
In the bowl of an electric mixer cream butter, granulated sugar, and brown sugar together until light and fluffy. Add eggs, one at a time, blending until smooth after each.
Sift together the flour, baking soda, baking powder, salt, and cocoa. In a separate small bowl, blend together the sour cream and rum. With mixer on low, alternately add the dry ingredients and the rum mixture, beginning and ending with the dry ingredients. Slowly pour in the water and mix until well-blended. Pour the batter into the pan and shake gently to level. Bake 30 to 40 minutes, until center of cake springs back when lightly touched. Remove from the oven and let cool for 5 to 6 minutes in the pan then invert onto a wire rack to cool completely.
When the cake is completely cool, invert cakes onto a serving plate. Using a hand held whisk, stir the cooled chocolate icing, adding the rum gradually. Once rum is incorporated, whisk vigorously for a few seconds until the icing is slightly lighter in color. Spread 2/3 of the icing over the sides and top of the cake. Take chocolate shaving from the freezer and adhere shaving to the sides of the cake. Spread the cherry preserves gently over the top of the cake in a heart shape design, leaving at least a 1-inch border around the edges. Use the remaining icing to make a decorative design around the cherry heart and around the top edge of the cake.