Method

1. Take the shortcrust pastry out of the fridge 45 minutes before using to allow it to come to room temperature
2. Preheat the oven to 200°C/gas mark 6. Lightly grease a 23cm square (or rectangular) tart tin with a removable base
3. Roll out the pastry to the thickness of a pound coin, making sure it is wide enough to line the tin completely with plenty of pastry to overhang the sides. Carefully lift the rolled pastry over the tin and gently press into the base and up the sides
4. Use a fork to prick the base a few times, then line the pastry with baking paper and weigh down with baking beans
5. Bake blind for 15 minutes, then remove the beans and paper and return the tin to the oven for another 5 minutes until the pastry is fully cooked through – it should be crisp and a golden biscuit colour
6. Leave to cool slightly, then trim off the excess pastry from the sides with a sharp knife to leave a smooth, neat edge
7. Reduce the oven temperature to 180°C/gas mark 4
8. Smooth the caramel over the base of the pastry – if too thick to use, whisk the caramel briefly in a bowl to make it easier to spread. Sprinkle the sea salt flakes over the top and set aside while making the chocolate topping
9. Place the dark chocolate and butter in a bowl over a pan of barely simmering water on the hob, ensuring that the bowl is not touching the water. Do the same with the white chocolate in a separate bowl
10. Allow both to melt down gently, stirring the dark chocolate and butter together so that they are fully incorporated. When both are melted, turn off the heat but leave the bowls sat over the water to stay warm
11. In a mixing bowl, whisk together the whole eggs, yolks and caster sugar with an electric whisk for 5 minutes. When the mixture has thickened and is pale in colour, pour in the dark chocolate and fold through to create an even mixture
12. Pour this into the tart tin to completely cover the caramel. Spoon lines of the melted white chocolate over the dark chocolate mixture and drag a cocktail stick through the white chocolate to create swirls
13. Bake the tart for 20 minutes until just set on top – the centre should still have a slight wobble when shaken
14. Leave to cool for an hour so that it is slightly gooey when cut. Slice into portions to serve, preferably with a handsome dollop of vanilla ice cream