Heat oven to 170ºC Butter two 9inch sandwich tins and line with non-stick baking paper. In a large bowl, beat all the sponge ingredients together until you have a smooth, soft batter.
Divide the mixture between the tins, smooth the surface with a spatula or the back of a spoon, then bake for about 25 mins until golden and the cake springs back when pressed. Turn onto a cooling rack and leave to cool completely.
To make the filling, beat the butter until smooth and creamy, then gradually beat in icing sugar. Beat in 1tsp vanilla extract if you’re using it. Once combined, gradually add the custard as you don’t want the buttercream to split. Spread half the butter cream over one of the sponges, sandwich the second sponge on top and spread the remaining buttercream on top.
To make the topping, add the sugar and enough water to cover the sugar to a saucepan and put onto a low heat. Keep stirring until all sugar is dissolved and it’s dark brown in colour, take off the heat and add the butter quickly, stirring continuously. Lay out a metal tray with greaseproof paper on top and pour the caramel straight out onto it, spreading quickly.
Leave to set, then ideally using a blender crush your caramel. If you don’t have a blender put the caramel into a few plastic bags and crush with a rolling pin.
Sprinkle the topping all over your cake. If you have butter cream left over you could pipe some rosettes on top round the edges.
