Grease and line a 23cm/9" round cake tin. Preheat oven to 170C/325F.
In a saucepan over low heat, heat chocolate, butter and water. Stir frequently until melted and smooth. Remove from the heat and cool slightly.
In a large bowl, lightly beat eggs. Gradually beat cooled chocolate mixture into eggs.
Whisk sugar and flour into chocolate mixture, continue beating until smooth and well blended.
Pour mixture into cake tin. Bake for 45 minutes or until skewer inserted into centre of cake comes out clean.
Turn cake onto wire rack to cool.
For truffle mixture: In a bowl over a pan of simmering water melt chocolate, stirring frequently. Cool slightly, then stir icing sugar, butter and cream until well blended.
For chocolate glaze: In a saucepan over low heat, heat chocolate and cream, stirring frequently until melted and smooth. Keep warm.
Cut cooled mud cake into layers.
Spread truffle mixture between layers with a palette knife. Reserve some mixture to finish round the edges of the cake.
Position cake on wire cake rack and pour warm glaze over the top.With a palette knife to spread glaze to cover top and side evenly.
Put remaining truffle mixture into a piping bag with a star nozzle. Pipe a decorative border around the edge of the cake.