Preheat the oven to 180⁰C, gas mark 4. Grease and line 2 x 18cm sandwich tins with baking parchment.
Place margarine, sugar and eggs into a bowl with the sieved flour, baking powder and cocoa. Add 2 tbsp of hot water and beat well together with an electric hand mixer for 1-2 minutes.
Divide evenly between the sandwich tins and bake in the oven for 25-30 minutes until well risen and springy to the touch. Remove from the tins and cool on a wire rack for around 30 minutes.
Melt the butter and chocolate with the milk in a bowl over a pan of simmering water. Sieve the icing sugar and cocoa into a bowl and using a wooden spoon, beat in the chocolate mixture along with 1 tbsp hot water. The mixture should be thick enough to spread onto the cake (if not, leave to cool a little).
Sandwich the sponges together with a quarter of the icing. Use the remaining icing to cover the sides and top of the cake. Decorate with the pieces of Cadbury Fudge.
Dip a palette knife in hot water to make the icing easier to spread.