Melt together chocolate and butter over low heat, stirring until smooth. Remove from heat and transfer to a mixing bowl.
Add sugar, mix for 1 -2 minutes until melted.
Beat in the eggs one at a time, mixing until smooth between each addition.
Stir in the vanilla extract, cocoa powder, and rum until well mixed.
Pour batter into the greased 8-inch baking pan and bake for 25-30 minutes at 190°C, or until a toothpick inserted into the center comes out relatively clean with a few crumbs. Do not over-bake or the cake will become dry. Allow cake to cool in pan for 5 minutes before inverting onto a serving plate.
Meanwhile, prepare the glaze: in a small saucepan, melt together chocolate and butter, mixing until smooth.