Chocolate cloud cake

1. For the cake:

2. Preheat oven to 180C. Line the bottom of a 23cm springform tin with baking paper.
Melt the chocolate in a double boiler, then add softened butter and stir to melt the butter into the chocolate and add 2-3 tea spoons of instant coffee.
Beat the 2 whole eggs and 4 egg yolks with 75g of the caster sugar until fluffy, then gently add the chocolate mixture, rum and orange zest.
In another bowl, whisk the 4 egg whites until foamy, then gradually add the remaining 100g of sugar and whisk until the whites are holding their shape but not too stiff.
Fold a dollop of the egg whites into the chocolate mixture to lighten, and then fold in the rest of the whites. Pour into the prepared tin and bake for 35-40 minutes or until the cake is risen and cracked and the centre is no longer wobbly.
Cool the cake in its tin on a wire rack; the middle will sink as it cools.
When ready to serve the cake, place the still tin-bound cake on a cake stand or plate for serving and carefully remove the cake from its tin. Don't worry about cracks or rough edges: it's the rustic, crater look you're after.