Melt the chocolate in a bowl over a pan of simmering water (see
Cream together the butter and sugar until they are pale and fluffy. Gradually beat in the eggs, sifting in a little flour and cocoa powder halfway. Fold in the rest of the flour and cocoa. Pour in the melted chocolate and add 2 tablespoons rum. Stir to combine.
Grease and line a 20 cm (8 inch) springform tin or daisy tin. Pour the mixture into the tin and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 30 minutes, then cover with foil and cook for a further 15 minutes. The cake is ready when a skewer inserted in the centre comes out clean.
Cool in the tin for 30 minutes. Turn the cake on to a wire rack to finish cooling, placed over a baking sheet. Drizzle with the remainder of the rum.
Melt the chocolate for the ganache. Stir in the butter and cream, then pour over the cake, using a palette knife to help smooth it over. Decorate the top of the cake with the golden chocolate buttons.