Heat oven to 160C/140C fan/gas 3. Grease two 20cm cake tins and dust with flour. Tip all the cake ingredients into a large bowl and beat well with an electric hand whisk or wooden spoon to give a smooth, thick batter consistency.
Divide the mixture between the tins and bake in the oven for 30-35 mins until it springs back to the touch and a skewer inserted into the centre comes out clean. Leave to cool in the tins for about 10 mins, then turn out onto a wire rack.
Meanwhile, make the truffles. In a bowl, gently melt the milk chocolate in the microwave or over a pan of simmering water. Stir in the butter. Put the biscuits in a sandwich bag and crush them to fine crumbs by bashing with a rolling pin. Mix into the chocolate and chill in the fridge for about 20 mins. When chilled, roll into 11 even-sized balls with your hands. Coat in sprinkles and set aside.
For the icing, pour the cream into a saucepan and place over a medium heat. Once hot, stir in the butter until melted. Break the chocolate into small pieces, tip into a medium bowl and pour over the hot cream mixture. Stir well until the chocolate melts into the cream. Cool for about 30 mins.
Once the cake is completely cool, sandwich the two sponges together with the jam in the middle. Spread the top and sides with the icing. Finish by placing the truffles around the outside of the cake.