Prepare the Dough: Begin by combining water, milk, sugar, and instant yeast in a mixing bowl. Let it sit until frothy, about five minutes. Gradually add bread flour and salt, mixing until a rough dough forms.
Incorporate Butter: Once your dough has formed, incorporate the cubed European-style butter by kneading it into the mixture until fully incorporated.
Laminate the Dough: Roll out your dough into a rectangle shape and place softened butter on one half before folding over the other half like a book cover. Roll out again gently and fold several times to create layers.
Chill and Shape: Wrap your dough in plastic wrap and refrigerate for at least two hours. Once chilled, roll out into triangles that will hold your chocolate filling.
Fill with Chocolate: Place chocolate batons along the wide end of each triangle before rolling them tightly towards the pointy end to form croissants.
Bake to Perfection: Allow your shaped croissants to proof until doubled in size before brushing with egg yolk mixture for shine. Bake in a preheated oven until golden brown and flaky.