Preparation: 

Preheat the oven at 180°C (gas mark 4). Mix the eggs, the caster sugar and the flour in a bowl.
Cut the chocolate into small pieces in a small pan and add 2 tablespoons of water. Melt on a low heat whilst stirring continuously. Add the butter gradually. Keep stirring. When the butter and the chocolate have melted, remove the pan from the heat.
Pour the chocolate-butter mix over the eggs, sugar and flour. Mix thoroughly.
Pour the mixture into a greased and floured mould. Cook for 15 to 25 minutes (less cooking time for a fondant centre).
Use sweets, candles, sparklers to decorate it…You could also make a stencil in the shape of a mouse and sprinkle the cake with icing sugar.