Make the salted caramel sauce: Add the granulated sugar to a medium heavy-bottomed saucepan and heat over medium heat. Stir continuously with a heatproof spatula or wooden spoon as the sugar starts to clump, then melts into a deep amber liquid. Carefully add the butter cubes. The mixture will bubble up - just keep stirring until the butter melts completely. Slowly pour in the double cream while stirring. It will bubble up again - keep whisking until the mixture is smooth and glossy. Remove from the heat and stir in the sea salt. Let it cool for 10–15 minutes in the pan before transferring to a heatproof jar. The sauce will thicken as it cools.
Preheat oven to 175°C (350°F). Grease and line two 6-inch deep tins.
Reduce cider: simmer 360ml (1½ cups) cider in a pan until reduced by half (about 10 minutes). Cool slightly.
Mix dry ingredients: flour, baking powder, soda, spices, and salt.
In a stand mixer, cream butter and sugars until light and fluffy.
Add eggs one at a time, then mix in vanilla.
Alternate adding flour and cider, beginning and ending with flour. Fold in grated apple.
Divide between tins and bake 35-40 minutes, until a skewer comes out clean. Cool completely.
For buttercream: beat butter until pale, add icing sugar, then caramel and cream. Beat until smooth.
Assemble: spread frosting between layers and pipe over the top. Drizzle extra caramel if desired.