Make the toffee sauce. Put the cream, butter and sugar in a small pot and heat until the butter has melted. Let it come to the boil then simmer until the sauce turns a golden colour. Stir in the salt, to taste. Let the toffee sauce cool before using – store any extra in the fridge.
Preheat the oven to 180C (350F). Grease and line three cake tins, dust with flour and shake any excess.
Peel and cube the apples into dice-sized pieces. Put in a bowl with the sugar and lemon juice and toss together. Set aside.
Add all the dry ingredients to the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed to combine. Make sure there are no hidden lumps of brown sugar!
Add the cubed butter and mix for a couple of minutes until the mixture resembles coarse sand.
Add the eggs, one by one, mixing well after each addition.
Drain the apples and reserve their juices.
Mix the quark cheese with the apple juices and add to the batter while mixing on medium-low speed. Stop and scrape the bottom and sides of the mixing bowl. Continue mixing for a couple of minutes until you have a smooth and thick batter.
Fold the chopped apples into the batter and divide between the prepared cake tins. Smooth the batter with a palette knife.
Bake for around 30 minutes, or until a skewer inserted in the centre comes out clean. If the cakes are browning too quickly, cover with some foil after 20 minutes. Leave the cakes in the tins for 10 minutes before turning out onto a wire rack. Let the cakes cool completely before frosting.
Beat the mascarpone, icing sugar and toffee sauce in a stand mixer until the frosting is smooth and forms firm peaks.
Brush some of the toffee sauce on the cake layers and then fill with a generous amount of the frosting. Sandwich together and then cover the entire cake with frosting.
Preheat the grill (broiler). Place the apple slices on a tray lined with baking paper and sprinkle with the sugar. Grill until the sugar caramelises and apple slices have softened - 3-5 minutes. Keep a watchful eye on them as they can burn easily. Cool before using.
Drizzle the cake with toffee sauce (reheat if it has set) and then decorate with the apple slices. Sprinkle with cinnamon and serve.
I made a smaller cake in a 18cm (7in) tin which I then sliced in half. This took 45-50 minutes to bake and I had enough batter leftover to make six large muffins (these needed 20-25 minutes).