Heat oven to 180C/160C fan/gas 4 and grease 2 x 22cm round cake tins, lining the bases with baking parchment and greasing the parchment too.
Put the cocoa in a small bowl with 100ml hot water. Whisk together until smooth, then set aside. Put the butter and chocolate in a heatproof bowl set over a pan of simmering water and melt gently together, stirring occasionally. Remove from the heat and allow to cool slightly.
In a large bowl, beat the sugar and eggs together using an electric whisk until thick and pale. With the blades still running, pour in the chocolate mixture, mixing until smooth and fully combined. In a separate bowl, mix together the flour and baking powder. Sieve the dry ingredients over the chocolate mixture and fold together until no lumps remain. Add the cocoa mixture and mix to combine. Divide the batter between your prepared tins and bake for 30-35 mins or until a skewer inserted into the middle comesout clean. Allow the cakes to cool in the tins for about 10 mins, then turn out onto a wire rack, peeling off the parchment, to cool completely.
For the icing, beat the butter in a large bowl with an electric whisk until light and creamy. Beat in the icing sugar a little at a time, then add the peanut butter, mixing until smooth. Add a pinch of salt and the cream, again mixing until smooth.
To assemble the cake, use a large serrated knife to slice each cake in half. Place the first round of cake onto a cake board or serving plate and top with a thin layer of icing. Repeat until all the cake layers have been used. To decorate, use the remaining icing to spread over the top and sides of the cake. Use a spatula or a palette knife to create a swirl pattern around the outside of the cake, and finish by pressing the pretzels onto the top and sides – there is enough to cover about half the sides, leaving the icing partially on display. Best served within 2 days of baking, but the cake will keep for up to 4 days.