Preheat the oven to 180C / 350F / gas mark 4
Mix the caramel in a bowl with the salt, making sure it is thoroughly mixed.
Pour the caramel into the pastry case and spread evenly, you won't need to use it all.
To make the filling, melt the chocolate with the butter in a bowl over a pan of simmering water. Remove and leave to cool a little. Whisk the eggs and sugar using an electric beater until the mixture is light and fluffy and has increased in volume. Stir in the chocolate and butter.
Pour the mixture over the caramel and place in the preheated oven or baking oven of the Aga. Bake for 10-12 minutes. Remove from the oven and place on a cooling rack. Leave until completely cooled and place on a serving plate. Decorate with a a light scattering of sea salt flakes and serve.
It is good served at room temperature though it is easier to cut neatly with a warm knife if you leave it in the fridge until ready to serve.