In a large mixing bowl, combine the flour, salt, sugar, and instant yeast. Mix well.

Add the warm milk and warm water to the dry ingredients, and mix until a dough forms.

Knead the dough on a floured surface for 10-15 minutes, until it becomes smooth and elastic.

Shape the dough into a ball and place it in a lightly oiled bowl. Cover with a damp cloth and leave to rise in a warm place for about 1 hour, or until doubled in size.

Once the dough has risen, roll it out on a lightly floured surface into a large rectangle.

Cut the cold butter into thin slices and arrange them in the centre of the dough.

Fold the dough over the butter to enclose it completely.

Roll the dough out again into a long rectangle, then fold it into thirds like a letter.

Repeat step 8 twice more, rolling and folding the dough each time. This will create layers in the dough and give the croissants their flaky texture.

Once the dough has been folded for the final time, wrap it in cling film and chill in the fridge for at least 6 hours, preferably overnight.

Preheat the oven to 200°C/400°F.

Roll out the chilled dough on a floured surface to a thickness of about 5mm.

Cut the dough into triangles and roll each triangle up tightly from the wide end to the pointed end.

Place the croissants on a baking sheet lined with parchment paper.

Brush the beaten egg over the croissants.

Bake the croissants in the preheated oven for 15-20 minutes, or until golden brown and puffed up.

Serve warm with butter and jam.