Heat oven to 180C/160C fan/gas 4. Butter and flour two 20cm cake tins. Mix the buttermilk (or yogurt) with 2 tbsp lemon juice.
Beat the butter and sugar until creamy, then add the lemon zest. Gradually add the eggs a little at a time, beating well after each addition. Fold the flour into the batter, followed by the buttermilk mixture. Spoon the batter into the cake tins and level the surface. Bake for 25-30 mins, or until a skewer inserted into the centre comes out clean. Leave to sit for 10 mins, then turn out onto a wire rack and leave to cool.
To make the caramel, put 1 chilli in a jug and add 100ml boiling water. Leave to steep for 10 mins (don’t leave it any longer or the chilli flavour will be too strong). Strain into a pan, then scatter over the sugar. Heat gently until the sugar melts, then boil until you have a light caramel. Take off the heat and quickly dip the remaining whole chillies in, then leave them to set on a piece of baking parchment. Put the pan back on the heat and stir the remaining lemon juice (from the cake mixture), reserving 1 tbsp, into the caramel to make a sauce.
To make the frosting, beat the butter until very soft. Beat in the reserved 1 tbsp lemon juice and the icing sugar, then fold in the soft cheese – don’t beat it or the mixture will soften too much. If it’s too soft, chill for 10 mins.
Drizzle a tiny bit of caramel over one cake, then add a layer of lemon curd and place the other cake on top. Spread the frosting over the top, drizzle over the remaining caramel and decorate with the chillies.