For the détrempe, fit a stand mixer with the hook attachment and stir the flour, water, milk, sugar, and yeast on low speed to blend, then add the salt. Increase to one speed higher and knead for about 4 minutes, adding the butter mid way through kneading – the dough should just clean the sides of the bowl. Shape the dough into a rectangle (it will be soft), place it on a parchment lined baking tray and cover with a tea towel and then plastic wrap. Let the dough sit out for 90 minutes, then chill for at least an hour, up to 8 hours.
For the beurrage, shape the butter into an 8-inch square. Chill until ready to use if preparing in advance, but pull from the fridge to soften. Ensure that the butter offers the same resistance (is the same consistency) as the chilled dough – while the temperature of each may differ, the “give” should be the same.
On a floured work surface, turn out the chilled détrempe and roll out to a square about 14-inches across. Place the beurrage in the centre of the square, but rotated so that the points of the butter square fall at the middle of each flat side of the dough. Bring the corners of the dough together, wrapping the butter like an envelope and gently pinch the edges. Roll the dough out into a rectangle about 20-inches long and fold the dough into thirds (this is called a single fold). Return the dough to the baking tray, cover with the towel and plastic and chill for at least an hour, up to 8 hours.
Repeat rolling and folding the dough into singles folds 2 more times, rotating the dough 90 degrees each time before rolling and chilling the dough for at least an hour and up to 8 hours before each fold. Let the dough rest for at least 4 hours and up to 12 hours after the final fold before using.
For 12 plain croissants, measure out about 20 oz of dough and store the rest in the fridge. On a floured work surface, roll out the dough to a rectangle about 16-inches by 12-inches. Cut the dough in half horizontally and then cut 6 triangles from each.
Make a 1-inch score on the short side of each triangle, and roll up the croissant from this side. Curve the croissant so that the point of the triangle is at the bottom and pointing the opposite direction as the curve in. If you wish, you can pinch the croissant ends together so they hold the curve shape. Place these on a parchment –lined baking tray, leaving at least 3 inches between each other and cover these with a tea towel and then plastic. Let the croissants rise for 2 hours.
Preheat the oven to 190°C. Brush the croissants with the eggwash and then bake them for about 15 minutes until a rich golden brown.
The croissants are best enjoyed the day they are baked. If you wish to prepare ahead, you can make the dough, fold it and let it rise, and then cut and shape the croissants. Freeze the croissants on a baking tray, then pack in a container. The croissants should fully thaw and proof for about 3 hours before then baking.