Sieve dry ingredients all together then gently fold in the flour to the egg mixture spoon by spoon.
Pour into a greased ring tin and bake in a preheated oven at 180oc for 20-30 minutes.
Check the cake is fully cooked by sticking a skewer into the middle of the cake. If it comes out with no mixture on it the cake is ready. If not, allow another 5 minutes to bake in the oven. Remove from oven and allow to cool completely in the tin on a wire rack.
For the chocolate filling, break up 150g milk chocolate and place in a bowl. Pour the water over and stir until all the chocolate is melted. Allow to cool then pour in the cream and whisk until thick.
To make the sauce, chop up the chocolate then put it in a heat proof bowl over a saucepan of hot water. Add the milk, stir until melted then remove from the heat and add the butter.
Slice the cake in three. Place the bottom on a serving plate, drizzle the sauce on the bottom of the sponge cake the spread the truffle cream on top. Repeat this step then spread the remaining sauce all over the cake. Place in the fridge for 1 hour before eating.
If you have any chocolate bars left over chop them up into bite-size pieces then put them in the centre of the cake or on top.