1Preheat the oven to 180C/160C Fan/Gas 4. Grease and base-line 3 straight-sided 20cm sandwich tins.
2Mix together all the dry ingredients. In a jug, beat the wet ingredients together with a fork, then stir into the dry mixture and mix well. Pour into the tins and bake for 25 mins until well risen and a skewer inserted into the centre comes out clean. Cool in the tins on a wire rack.
3To make the ganache, bring the cream to the boil in a small pan, then immediately remove from the heat. Break the chocolate into the pan and leave to melt before stirring until smooth. Leave to cool and thicken.
4For the frosting, put the butter, soft cheese, icing sugar and peanut butter in a bowl and beat together until light and fluffy.
5Beat the cooled chocolate ganache until light and fluffy. Put one sponge flat-side up on a serving plate and spread with half the ganache. Spread over 4tbsp of the peanut-butter frosting. Top with a second sponge, flat-side down, and repeat, then top with the final sponge. Spread half the remaining frosting thinly all over the top and sides of the cake. (You may get crumbs in the frosting but don’t worry.) Chill until firm – this may take 1-2 hrs.
6Spread the remaining frosting all over the cake, smoothing with a palette knife. Chill until required.
7The topping involves making caramel nuts, which isn’t hard – just fiddly and a bit messy! Before you begin, put a sheet of newspaper on the floor to catch any drips. Next, for every macadamia, cut a 5cm length of sticky tape and keep to hand. Spike each nut with a cocktail stick.
8Slowly heat the caster sugar in a saucepan until melted, swirling it around from time to time. Slowly bring to the boil and simmer until the caramel is a deep golden brown; set aside for about 10 min to cool and thicken slightly.
9Dip a macadamia into the caramel and allow it to drip off the nut. If the mixture is too warm, the caramel will just run off the nut; if it’s the right temperature, it will cover the nut and drip off slowly, leaving a long thread of caramel that will harden as it cools. Tape the cocktail stick to the edge of the worktop, with the caramel ribbon hanging over the edge and above the paper.
10Repeat with the remaining nuts and leave to set. If you struggle with this step, just tip your nuts into the caramel, give them a stir and turn out onto a sheet of baking paper, separating them before they cool.
11When you are ready to serve, stand the jar of caramel sauce in a pan of hot water to warm it slightly. Give it a good stir then spread half over the top of the chilled cake; drizzle the rest down the sides.
12Scatter the chopped nuts and popcorn, if using, around the top edge of the cake. Carefully remove the nuts from the sticks and arrange on the cake. Serve immediately.