When your sourdough starte is at it's peak, make the main dough for pain au chocolate. In a bowl of your stand mixer, fitted with a dough hook attachment, mix the strong flour, salt and sugar. Pour in the water, milk and active sourdough starter. Turn the stand mixer on and knead for 5 minutes at medium speed. Leave the dough to relax for 15 minutes.

Run the stand mixer again on medium speed, and start adding 50g of room temperature butter a little bit at a time. Keep the mixer running for around 8-10 minutes until the butter is fully incorporated and you have an elastic cohesive dough. It shouldn’t be sticky at this stage.

Shape a tight ball, slash the dough with a sharp knife (a razor or a dough scorer) in a cross on top of the dough (see photos above). Place the dough in a lightly greased bowl. Cover and leave in a warm spot to bulk prove for approx. 4 hours: your dough should rise x1.5 times. Place the dough in the fridge for another 3-4 hours to finish proving and double in size.

In the meantime, take the butter out of the fridge in advance so it is soft enough to be rolled. Place it on a large sheet of baking parchment, place another sheet of parchment on top. Using a rolling pin, gently bash to flatter the butter. Then fold the baking parchment under into a neat 18x18cm (7×7 in) square and roll the butter inside the parchment parcel into all four edges, aiming for the same thickness across (for process photos see text above). Enclosing the butter into the baking parchment square will help you get an even butter square with sharp edges. Still wrapped in the baking parchment, place the butter in the fridge to firm up.

Take the dough out of the fridge and place it onto a floured worktop. Knock the air out with the bottom of your palm and roll it into a square around 28x28cm (11x11in). Try to make it even.

Unwrap the butter (keep the baking parchment). Place the butter in the middle of the dough on the diagonal (see the photos above).

Taking one side at a time, fold the edges of the dough into the middle creating a square envelope. Once you fold all four sides into the middle, pinch the edges together. You should now have a 20x20cm (8x8in) butter parcel. Wrap it in baking parchment and freeze on a flat surface for 10 minutes.

First Fold: Take the parcel out of the freezer, unwrap and roll the parcel out into a rectangle that’s around 50x20cm (20x8in). Make sure your dough isn’t sticking to the worktop as you roll: you want the butter encased and the dough may tear if it sticks. Dust with flour if needed. Using a sharp knife or a pizza cutter, cut off any untidy short edges that don’t have any butter in them.

Fold the rolled-out dough like a letter: from the short edge, hold ⅓ of the sheet into the middle, then take the opposite short edge and fold it over the first one. Wrap the dough in the baking parchment you kept the butter in, or cling film, and place in the freezer for 15 minutes, then transfer to the fridge for 1 hour.

Second Fold: Take the dough out of the fridge and place it short edge in front of you. Roll the dough out to 50x20cm (20x8in) again. Fold the top third towards the centre and then the bottom third over (just like before). Wrap and place the pastry in the freezer for 15 minutes, then fridge for 1 hour.

Third Fold: Just like before, roll out the dough to 50x20cm (20x8in) rectangle. Fold like a letter, wrap and place in the fridge for a minimum of 6 hours (we leave it in the fridge overnight).

Take the pastry out of the refrigerator and let it warm up for 5 minutes at room temperature. Unwrap and, on a lightly floured worktop, roll the pastry out into a 60x40cm (24x16 in) rectangle, gently getting all the air out of the pastry. You don’t want pockets of air to burst abruptly though, exposing the butter.

Trim the pastry from all four sides to create even edges. Using a sharp knife or a pizza cutter, cut the pastry sheet in half lengthways. Then each long strip into 6 rectangles measuring 20x10cm (8x4in). You should have 12 even rectangles (see photos above).

Boil a kettle of water. Pour the water into an ovenproof dish and place it in the bottom of your cold oven. Close the oven door and let the water steam inside.

Line two large baking sheets with baking parchment.

Taking one rectangle at a time, place a line of chocolate along the short side, about 2-3cm away from the edge. Fold the pastry over the chocolate. Then add another line of chocolate next to the fold. Continue to roll until you have a neat spiral. Repeat with the rest of the rectangles. Place the sourdough pain au chocolat seam side down on the prepared baking sheets spaced out, leaving enough space for them to rise.

Your oven should be warm and humid by now. The optimal temperature should be around 23-24 C (any higher and your butter will start melting into the pastry). Cover the pastries with a large sheet of lightly oiled clingfilm.

Place the sheets with sourdough pain au chocolat in the turned-off oven for 3-4 hours to prove. The pastries should puff up significantly and the layers should be clearly visible.

Once your pains au chocolat have proved, place them in the fridge for 15-20 minutes.

Whilst the sourdough pain au chocolat are chilling, take the ovenproof dish with water out of the oven and then preheat it to 190°C Fan.

In a small bowl, whisk the egg yolk and cream (or milk). Using a pastry brush, very gently brush the pastries with the mixture.

Bake the pastries for around 25-30 minutes. Leave the sourdough pain au chocolat to cool for 30 minutes. They are best served within 2 hours from baking! We enjoy them most when they are still warm and the chocolate is oozing out!