Combine the flour, salt, sugar, 30 g of butter, water, milk and yeast in the bowl of a stand mixer.Knead for 4 minutes at medium speed. Then knead for 2 minutes at high speed.
Shape the dough into a ball and let it rest for 30 minutes.
Knock the dough back and shape it into a rough rectangle of about 24 x 12 cm.
Wrap it in cling film and chill for at least 1 hour 30 minutes.
Prepare the butter block: fold a 17 x 14 cm rectangle out of baking paper to act as a mould. Beat the butter inside the rectangle with a rolling pin so it takes that shape.
Take the dough out and roll it out widthways so it can wrap around at least twice the butter block.
Fold it over and seal both edges of the dough to close the rectangle.
Even the dough out gently with the rolling pin.
Do not hesitate to put the dough back in the fridge if your kitchen is warm.
Roll the dough out into a long rectangle. The thickness must be even throughout.
Give it a double turn.
Wrap and chill for at least 1 hour 30 minutes.
Roll the dough out into a long rectangle again. Watch the orientation of the dough: the folds must be perpendicular to the edge of your work surface.
Give it a single turn.
Wrap and chill for at least 1 hour 30 minutes.
Roll out to a 30 x 28 cm rectangle, 5 mm thick.
Trim the uneven edges.
Cut out your dough triangles, cutting every 8 cm.
Make a small cut in the middle of the base of each triangle and roll it up.
Arrange the croissants staggered on the tray (if you can).
Let them prove for 1 hour 30 minutes in a switched-off oven with a pan of hot water below the rack.
Glaze the croissants with an egg yolk and a little milk.
Preheat your oven to 200°C.
Put the croissants in and, as soon as they are in the oven, lower the temperature to 170°C for 20 minutes.
For two-tone croissants, follow the video below.