Grease and line the bottom of a 23cm square tin. Pre heat the oven to 180C / 160C fan.
Add the flour, butter, sugar, milk, cocoa powder, baking powder and bicarbonate of soda to a large mixing bowl. Use an electric whisk to beat, the mixture should drop easily from a spoon.
Pour out into the prepared tin and bake for 25 minutes. The cake is ready when the top is slightly firm to the touch and the edges of the cake are pulling away from the tin.
Leave to cool in the tin for 10 minutes and then carefully turn out onto a wire rack. Allow to cool completely.
Cut the traybake into 20 squares. Half the truffles and place a half, face down on top of each cake.
Prepare the icing by melting the dark chocolate and margarine together in the microwave. Whisk in the sieved icing sugar. The mixture should be a pourable consistency.
Pour over the sponge squares, using a palette knife to eaten the edges. Melt the milk chocolate, then use a piping bag or spoon to drizzle over the sponges. Leave to cool at room temperature, don’t chill in the fridge as this will dull the chocolate icing and you want these fancies to be shiny and inviting!