Preheat the oven to 160 degrees Celsius. Grease the baking tin with butter. Melt the butter with the chocolate in a bain-marie or carefully in the microwave. Stir occasionally. Beat the eggs and sugar in another bowl, then stir in the flour. Mix the egg mixture with the chocolate mixture and stir very well (for at least 5 minutes). The longer you beat the mixture, the lighter the cake. Pour the mixture into the baking tin and bake the tart for about 20 minutes until cooked. Allow the tart to cool, remove from the tin and slice horizontally.
Mix the jam. Mix half the rum with the jam. Sprinkle the remaining rum over the two halves. Coat the lower half of the tart with the jam/rum mixture and place the top half over it. Decorate the tart with whipped cream as desired.