Set 6 physalis aside to use as a garnish, then remove the remaining berries from their papery cases. Cut in half, then add to a saucepan with 100g caster sugar and the lemon juice. Simmer for 8-10 minutes, or until the physalis are soft and pulpy. Leave to cool for 5 minutes, then blitz the pulp in a food processor. Pass the liquid through a sieve into a clean saucepan, then return to the heat.
Mix the cornflour and 1 tablespoon of the physalis syrup together in a small cup or bowl, then stir into the pan. Simmer on low for until thickened, then divide between the 6 ramekins. Chill in the freezer for 30 minutes.
Heat the oven to 140°C/ 120°C fan/ Gas mark 4. Heat the cream, the crème fraiche and milk in a saucepan - take it off the heat just before it comes to the boil. Whisk the egg yolks, vanilla extract and remaining sugar together in a large bowl until light and fluffy, then gradually add the hot cream, continuing to whisk as you do so. Pour through a sieve into a jug, then skim off any froth that settles on top.
Sit the ramekins in a large, deep-sided roasting tray, then divide the hot cream equally between the ramekins. Pour hot tap water around the dishes, bringing the level to about 15mm below the rims, then bake for 35-40 minutes. The set cream should have a jelly-like wobble when given a shake; careful that you don’t overcook them.
Lift the ramekins from the water (use a cloth as they’ll be hot), then let them cool on a wire rack for 15 minutes. Transfer them to the fridge and leave to cool completely - overnight if possible.
Sprinkle 1 teaspoon of Demerara sugar over each of the brûlées, then caramelise with a blowtorch or under a hot grill. Leave the caramel to harden for about 45 minutes then serve, with physalis berries on the side.