Preheat your oven to 180 C/ 350 F. Spray 3x15cm (6in) cake tins with cake release and line the bottoms with baking paper.
Put all the ingredients for the apple purée in a saucepan and bring to the boil over medium-low heat. Keep stirring until the apples are soft and breaking up. Use an immersion blender or food processor to process to a smooth purée.
Measure 300g (10 1/2oz) of purée and reserve the rest to be used between the layers.
Add the vegetable oil and eggs to the 300g (10 1/2oz) apple purée and pulse until liquidised.
Sift the dry ingredients in a large bowl. Add the wet ingredients and fold together with a spatula, making sure you scrape the bottom and sides of the bowl.
Divide the batter between the prepared cake tins.
Bake in the centre of the oven for about 22-25 minutes. The cake is ready when firm on top and a skewer inserted in the centre comes out clean.
Leave to cool in the tins for 10 minutes and carefully turn out onto a wire rack to cool.
Mix the mascarpone, icing sugar, spices and vanilla on low speed, gradually adding the cream. Gradually increase the speed until you have firm peaks, scraping the down the bottom and sides of the bowl halfway through. Do not over beat as the frosting may curdle. Transfer to a piping bag fitted with a round tip.
Transfer the remaining apple purée into a smaller piping bag fitted with a small round tip.
Pipe a ring of frosting around the edge of the bottom cake layer.
Pipe apple purée inside the ring and level. Ad a full layer of frosting over the top (see video for demo) and level. Repeat until you have assembled the cake.
Pipe or smooth a thin layer of frosting (crumb coating) over the entire cake. Use a palette knife to smooth the top of the cake letting the buttercream spread over the edges.
Use a side scraper to smooth the buttercream on the sides of the cake – using a turntable will make this easier. Freeze the cake for 30 minutes.
Prepare the toffee sauce. Put the butter and sugar in a small saucepan and whisk together over medium heat until the butter is completely melted and sugar has dissolved.
Add the cream (take care as the toffee might sputter), vanilla and salt and boil for one minute. Leave to cool slightly.
Add another layer of frosting to cover the cake. Use a serrated side scraper to smooth the frosting (or a plain one for a smooth finish).
Carefully drizzle some toffee sauce around the perimeter of the cake so that it drizzles down the sides.
Top with a toffee apple or pipe any remaining frosting over the top of the cake to decorate. Add a few chopped nuts around the base of the cake to decorate (optional).
You can use store-bought caramel sauce instead of making the toffee sauce. This recipe will make quite a lot of toffee sauce – you can add some between the layers or keep it in the fridge to drizzle over ice cream or other desserts.