Heat oven to 180C/160C fan/gas 4. Grease a 2.5-litre bundt tin (ours was 26cm in diameter, available from nordicware.com) with some melted butter, making sure you get in all the crevices. Mix the cocoa with 50ml kettle-hot water and set aside to cool.
Mash the bananas, then stir in the butter, milk, eggs and vanilla. In another bowl, combine the flour, bicarb, sugar and 1/2 tsp salt. Shake the bowl a few times to encourage any lumps of sugar to come to the surface, squeeze them through your fingers and mix again.
Stir the banana mixture into the dry ingredients, then transfer half the cake mixture to another bowl. Add the cocoa mix, 50g yogurt and the chocolate chips to one bowl, and the remaining 100g yogurt and 50g peanuts to the other.
Scrape the peanut mixture into the prepared tin, then spoon over the chocolate mixture. Swirl a skewer through the two cake mixtures to create a marbled effect. Bake on the middle shelf for 1 hr. Check the cake is cooked by inserting a skewer into the centre of the sponge – if it comes out coated in any wet mixture, return to the oven for 10 mins more, then check again. Cool for 15 mins in the tin, then transfer to a wire rack to cool completely.
Mix the caramel with 1 tbsp milk to create a drizzly icing. Once the cake is cool, drizzle over the icing and scatter with the remaining peanuts. Will keep in a tin for four days.