Photograph: Ria Osbourne
First make the caramel. In a wide pan, heat 150ml water together with 100g caster sugar until the sugar has dissolved, then bring it to the boil and stir until it's a dark golden colour, then add the crème fraîche and stir on a low heat until smooth.
Preheat the oven to 200°C, fan 180°C, gas 6. Melt 1 tbsp butter and use a pastry brush to grease a 20cm x 10cm deep round ceramic baking dish. Dust with caster sugar, ensuring you have no gaps, and shake off the excess.
In a medium saucepan, off the heat, whisk together the egg yolks, caster sugar and flour, then slowly add the warm milk, whisking to incorporate. Cook on a medium heat, whisking constantly, until thick, then stir in the caramel. Pour into a large bowl and allow to cool slightly until it's just warm to the touch, whisking occasionally to prevent a skin forming.
Whisk the egg whites with the vinegar until stiff peaks form. Mix a spoonful of the egg whites into the caramel-egg mixture to loosen, then carefully fold in the rest. Pour into the prepared baking dish and tap hard once on a level surface to remove air bubbles. Bake for 40-45 minutes until golden and risen with a slight wobble in the centre.
While the soufflé is baking, make the salted caramel sauce. Put all of the ingredients in a small pan and heat, stirring. Bubble for 1-2 minutes until glossy. Serve the soufflé straight from the oven with the salted caramel sauce on the side for drizzling over (you'll need to serve the soufflé immediately as it does sink quite quickly).