Make the dough in a stand mixer. Combine the flour, salt, sugar and 20g softened butter. Dissolve the yeast in around 80ml tepid water, around 38C, not too hot or it will kill the yeast. Add 20g of beaten egg and the milk to the stand mixer bowl. Pour in the yeast-water mixture and turn on to a low speed with a dough hook attachment for approximately 5 mins until the dough comes together to form a rough ball. Increase the speed to medium-high and knead for a further 6-10 mins until the dough is smooth. The exact time will depend on your stand mixer.

Wrap the dough tightly and rest at room temperature for 10 mins. Unwrap and roll out the dough into a rectangle around 4mm thick, about 40 x 30cm. Wrap the dough rectangle and put in the freezer for 1 hr to rest or in the fridge overnight.

Meanwhile, on a sheet of baking parchment, shape the remaining 200g butter into a flat rectangle (about 20 x 30cm) using a rolling pin. It should be around half the length of your dough rectangle, but the same width. Put in the fridge until needed – you want it to be cold but flexible.

Take the chilled dough out of the fridge or freezer and lay the butter in the centre. Fold the dough edges in to meet at the centre, covering the butter. Rotate the dough 90 degrees and roll it out to about 40cm in length, keeping the same width. Handle the dough gently and return it to the fridge for 10-20 mins if it starts getting too warm.

Repeat the process of folding the edges of the dough into the middle, then fold it over on itself again like a book (this is known as a double turn). Chill for 30 mins.

Rotate the dough 90 degrees again. Roll it out long again and fold the edges in to meet in the middle. Chill for a further 30 mins.

Roll out the dough to 32 x 30cm and cut 8 rectangles, 8 x 15cm each. Put a chocolate baton on one long edge of each rectangle, fold the dough inwards, then put the second chocolate baton next to the folded dough and roll the dough around the encase the chocolate, with the seams of the dough now facing down.

Prove the pain au chocolat at 26-29°C (this is really important as proving at any higher temperature will mean the butter between the layers melts) for around 2-3 hrs, until doubled in size.

Heat the oven to 175C/155C fan/gas 4, brush the pain au chocolat with the remaining egg and bake for 15-18 mins until golden brown and cooked through. Best eaten on the day but will keep in an airtight container for two days.