Preheat the oven to 170°C. Grease and line a 7 inch springform cake tin with nonstick baking paper. Melt together the sugar, chocolate and butter in a saucepan until smooth. Remove from the heat, allow to cool slightly, then stir in the brandy if using.
Beat the eggs in a large mixing bowl until smooth, then beat in the flour. Slowly beat the melted chocolate mixture into the eggs and flour until smooth. Sit the lined cake tin in a large roasting tin and wrap a double layer of tin foil around the outside of the cake tin so that it comes up past the rim of the tin. Pour the chocolate batter into the tin and tap lightly on the side to release any trapped air bubbles.
Pour boiling water into the roasting tin so that it comes about halfway up the side of the cake tin. Bake in the oven for 40-50 minutes until just set; the tip of a knife will come out just clean when inserted about 3 inches from the edge of the cake. Remove from the oven and carefully lift out of the roasting tin. Allow to cool before turning out from the springform tin. Carefully lift onto a presentation plate. Dust the cake liberally with the icing sugar and decorate with birthday candles before serving.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.