To make the brioche dough Place the flour, salt, sugar and yeast in the bowl of your Kenwood mixer fitted with the dough hook, keeping the salt and yeast apart. Add the eggs and mix on the lowest speed for 5 minutes until completely absorbed. Increase to the next speed setting and mix for another 5 minutes until the dough comes away from the side of the bowl. Now add the cubes of butter (*see note 2 below) and continue to mix for 2–3 minutes until fully incorporated.
Remove the mixer bowl from the machine and cover the brioche dough loosely with a tea towel to prevent the surface from drying and to stop a crust forming. Leave to rise at room temperature for 1 hour. Then refrigerate the dough for 1 hour (*see note 3 below).
To make the crème fraîche filling In a large bowl, mix all the ingredients together until evenly combined, then cover and reserve in the fridge.
Lay a 35cm square of greaseproof paper on your peel (or tray) and sit the tart ring on top (*see note 4 below). Place the dough in the middle. With lightly floured hands, flatten the brioche dough, pushing it from the middle to the outside until you have an even thickness of 3–4mm and a raised rim of about 1cm. Leave to prove at room temperature for 25 minutes (*see note 5 below).
Preheat the oven to 180°C fan and place a baking stone or tray on the middle shelf.
Knock back the dough base with your fingertips, but only up to the edge of the raised rim (*see note 6 below). Brush the rim with egg wash. Pour three-quarters of the crème filling into the brioche case. Carefully slide the gâteau onto the baking stone or hot baking tray and pour in the rest of the filling.
Bake for 10 minutes, then sprinkle caster sugar over the surface and bake for a further 15 minutes until golden brown and cooked.
Remove the ring and transfer to a wire rack to cool slightly. Serve warm.
1 If fresh yeast is unavailable then use dried yeast, but halve the quantity.
2 Adding the butter in small pieces at room temperature enables it to be incorporated quickly into the dough, producing a silky smooth texture.
3 You will need to chill the dough at this stage, otherwise it will be too soft to work with. Chilling will also slow down the proving.
4 A ring mould isn’t essential. You could simply shape the round with your hands and let the gâteau take its own shape, flattening the base until you have the desired thickness and creating the rim as described.
5 This second proving will help to develop the flavour and puff up the dough.
6 You will need to knock back the dough in the centre to create a well for the crème fraiche mixture.