Grease and line two 8 inch cake tins. Preheat the oven to 180C
Mix the flour, cocoa, sugar and baking powder together in a large bowl. Make a well in the centre of the mixture and add the wet ingredients, stirring until only just combined.
If bubbles start to rise to the surface, tap the bowl sharply on the worktop to slow down the baking powder. Pour half the mixture into each tin and tap again if necessary.
Bake for 20 minutes in the centre of the oven. When done the sides should start to come away from the tins, the tops shoud feel slightly springy and a toothpick inserted into the middles should come out clean.
Cool in the tins for 15 minutes then transfer to a wire rack. Once fullly cooled, level the tops of the cakes with a large serrated knife. Enjoy the offcuts with a cup of tea.
Whip the peanut butter, shortening and vanilla extract until light and fluffy. Add the icing sugar a little at a time until you reach a thick but spreadable consistency. You may need to thicken slightly with more icing sugar or thin with a little soya milk, depending on the brands of fat and peanut butter used.
Sandwich the cakes together with a dollop of icing and use a spatula to spread to the edges. Use the rest of the icing to cover the cake smoothly.
Heat the soya milk in a small pan until almost boiling. Remove from the heat and add the chocolate chips, stirring well as they melt.
Before the mixture cools completely pour it over the cake, allowing some to drip down the sides however you like.
Decorate with your favourite toppings - we used chocolate chips, peanuts and popcorn but a peanut brittle would be fantastic!