Grease and line the bottom of 3 x 20cm (8 inch) sandwich tins. This is important as you need to get the batter into the oven as soon as the wet and dry ingredients are combined.
Make the peanut butter chocolate sponges
Put the milk chocolate and dark chocolate in a heatproof bowl. Melt in the microwave in 30 second bursts, stirring between each burst until just melted.
Stir in the smooth peanut butter and allow to cool for 5 minutes.
Add the eggs, buttermilk, vanilla, butter and sugar. Whisk until everything is combined.
Sift in the flour and bicarbonate of soda. Fold together - you'll see the bicarbonate of soda (baking soda) react with the acidic buttermilk pretty much straight away
Divide the mixture between the three prepared tins and level off.
Put the tins in the oven straight away, aiming to position towards the centre of the oven. Bake for 25-30 minutes until risen and firm to the touch. These cakes tend to rise quite level - they shouldn't dome.
Allow the cakes to cool in their tins for 5 minutes then turn out onto a wire rack to cool completely.
Make the peanut butter chocolate ganache
Put the milk chocolate and dark chocolate in a heatproof bowl and melt in the microwave in 30 second bursts.
Stir in the smooth peanut butter and set aside to cool until only slightly warm to the touch. The peanut butter will have helped cool the chocolate, so this should take about 10 minutes
Whilst whisking the chocolate peanut butter mixture, gradually pour in the cream. Once all of the cream is added, you should have a thick and glossy peanut butter chocolate ganache.
Assemble the peanut butter fudge cake
Place the first sponge on a board.
Top with one third of the peanut butter chocolate ganache, spreading just shy of the edges (it will spread more when the subsequent layers are placed on top).
Put the second cake on top and top with a further a third of the peanut butter chocolate ganache.
Position the third and final cake on top Pile on the remainder of the peanut butter chocolate ganache, using your spoon of a palette knife to add swirls and peaks.
Loosen the peanut butter with a very brief burst in the microwave - 10 to 15 seconds should do it. You don't want it to be hot as that will melt the ganache.
Drizzle the peanut butter all over the top of the cake, then swirl again to lightly combine the peanut butter with the peanut butter chocolate ganache.
For neat slices, put the whole cake in the fridge to firm up for 30 minutes before serving.