In a large bowl, mix bread flour, instant yeast, sugar, and salt.
Add the egg and milk, stirring until the dough begins to form.
Knead for about 5-7 minutes until smooth and elastic.
Add 20g of unsalted butter and continue kneading until fully incorporated.
Cover and let the dough rest for about 1 hour, or until doubled in size.
Flatten the 90g of butter between two parchment sheets into a thin rectangle and refrigerate until firm.
Roll out the dough into a rectangle larger than the butter sheet. Place the butter in the center and fold the dough over it.
Roll the dough into a large rectangle and cut it into triangular shapes.
Roll each triangle from the wide base to the tip to form croissants.
Arrange the shaped croissants on a baking tray and let them rise for 45 minutes.
Preheat the oven to 180°C (350°F).
Brush the croissants with an egg wash (1 beaten egg mixed with 1 tbsp. milk).
Bake for 18-20 minutes until golden brown and flaky.
Let them cool slightly before serving.