First make the sponge for the roulade. Whisk the eggs and caster sugar in a stand mixer for approximately ten minutes until pale, fluffy and thick.
Preheat your oven to 180 degrees (fan).
Add the vanilla extract then fold in the flour thoroughly.
Line a Swiss roll tin with baking parchment (I used foil backed parchment). Pour the mixture into the lined tin, make sure it is spread to the corners and level.
Bake for approx. 13 to 15 minutes until cooked in the centre, golden brown and springy to the touch.
While the cake bakes, make the filling. Peel and core the cooking apple. Cut about six to eight thin slices and set aside. Chop the rest into small pieces and put it in a bowl with the dark muscovado sugar. Microwave on high for three minutes, stirring after each minute. Leave it to cool.
Put the apple slices in a loaf tin, sprinkle with a little muscavado sugar and bake at 180 degrees for ten to fifteen minutes until golden.
Beat the softened butter and icing sugar for five minutes in a stand mixer, then add three tablespoons of caramel sauce. Mix well.
Once the cake has cooled, unroll it and spread the buttercream inside. Add the cooked apple mixture on top of the buttercream. Roll up the sponge again tightly with the filling inside.
Put the caramel sauce in a bowl and stir to soften it. Drip the caramel sauce over the top of the cake to give a drippy ganache look.
Arrange the apple slices on top and drizzle with a little more caramel sauce.
Slice and serve!