For the salted caramel,scatter the sugar across the base of a large saucepan and place over medium-high heat. Cook for two to three minutes or until golden,stirring with a spoon to ensure the sugar on the bottom doesn't burn. Continue to cook until desired consistency is achieved,being careful as sugar can burn quickly,this can be up to another two to three minutes.
Remove from the heat,gently and slowly pour in the cream and using a whisk,whisk vigorously. Add the cold butter,a few cubes at a time,whisking continuously to ensure the sauce doesn't split.
Whisk in the salt then pour the caramel into a heatproof container and allow to thicken slightly and cool completely in the fridge (at least 30 minutes).
While the caramel is cooling,prepare the apple filling by adding the apple pieces,vanilla bean extract,sugars and a squeeze of lemon juice to a medium-sized saucepan. Grate over the lemon zest,cover with the lid and cook over medium-low heat,stirring occasionally and using a wooden spoon to break up the apple as it cooks,until the apples are tender and soft (about 30 minutes). Cook the apple longer than you might for a pie or crumble filling,given this will only have the few additional minutes in the toaster. Allow to cool.
Place the brioche slices on a flat surface. Add half a tablespoon of the cooled caramel (it will have almost a fudgy consistency) to the centre of one slice. Add two to three tablespoons of apple mixture and top with an additional piece of brioche. Repeat with remaining ingredients.
Gently transfer sandwiches to a sandwich press and toast for one to two minutes or until the ingredients have warmed through and the bread is golden.
Serve immediately with vanilla ice-cream and warmed-through leftover salted caramel.
*I used a combination of tart granny smith and sweet royal gala apples.