Break the eggs into a heatproof bowl, add the sugar, and place over a pan of simmering water. Whisk with an electric whisk until the mixture has doubled in volume and leaves a ribbon trail on the surface when you remove the whisk.
Combine the melted butter and vanilla extract and fold into the mix.
Sift in the flour and cocoa, gently fold until combined. Pour the batter into the prepared cake tin and bake in the preheated oven, just above the middle shelf, for 25 minutes or until firm to the touch and a skewer comes out clean when inserted into the middle of the cake.
Remove from the oven and leave to cool in the tin for 5-10 minutes. Then remove from the tin and cool completely on a wire rack.
*Heat the cream to boiling point, leave to cool for 7 minutes before adding the chocolate pieces, stir, once glossy chill for a least 40 minutes or until it starts to thicken.
Chill the sponge in the freezer for 20 mins until quite firm but not frozen, this will help you to cut 3 equal horizontal slices.
Whisk your butter and peanut butter for 3 minutes, add the icing sugar and whip for a further 3 minutes until light and fluffy. Spoon into a piping bag fitted with a small open star nozzle.
Whisk the ganache with an electric hand whisk or free standing mixer until it looks light and airy and has doubled in volume and spoon into a disposable piping bag.
Lay the first layer of chilled sponge onto a serving plate, pipe lines of ganache across it, sprinkle with a quarter of the frozen snickers. Repeat with the second sponge, then top with the third. With a palette knife spread the remaining ganache thinly over the sides and top of the cake. Pop in the fridge to chill until firm.
Once the sponge is very firm cut into 12 equal pieces and pipe buttercream rosettes over the top and sides of the individual cakes, sprinkle with the remaining snickers to serve.
*By boiling the cream for your ganache this pasteurises the cream taking away the need to store in the fridge. But if you add the chocolate when the cream is still boiling it will split and ruin.