To Make the Croissant Dough: In a small bowl, combine your water, milk, sugar, and yeast. Stir to combine, and let sit for 5 minutes, or until bubbling and foamy on top.
In a separate large bowl, whisk together your flour and salt. Add your frozen butter to the flour, and toss to distribute the butter and coat the butter in the flour. Add your yeast mixture to the flour / butter mixture. Stir until everything comes together into a dough. Don't knead! ** (see notes)
Transfer your dough to a large piece of plastic wrap. Wrap tightly and chill in the fridge for at least 1 hour, or overnight
Laminating the Croissant Dough: Take your chilled dough out of the fridge and unwrap. Place on a lightly floured work surface, and roll the dough out into a rectangle that's about 9 x 15 inches large. Starting from a short end of the dough, roll the dough up tightly into a tube (like you would for cinnamon rolls). Rotate the dough 90°, and roll the dough out into a 9 x 15 inch rectangle again. Roll the dough into a tube once more, then wrap the dough in plastic wrap and refrigerate for another hour or up to 12 hours.
Shaping the Croissants: Take your chilled dough out of the fridge and place on a lightly floured work surface. To make shaping easier, I like to cut my dough in half and keep the other half in the fridge while I work to keep the butter cold. Roll the dough out into a rectangle that's just under a ¼ inch thick. Cut off the sides of the dough to create straight edges, then cut triangles out of the dough.
Take one triangle and cut a short ½ inch slit on the bottom of the triangle. This helps with expansion while the croissants bake! Starting from the bottom of the triangle, roll the dough into a tube. Repeat the rolling and shaping with the remaining dough.
Transfer your shaped croissants (seam side down) onto a large, parchment lined baking sheet. Make sure to leave about 1 inch between each croissant to give them room to expand. Cover the croissants and let them sit and rise in a warm place for 30 minutes, or until just under doubled in size. Preheat your oven to 400°F.
In a small bowl, beat together your egg yolk and cream. Brush this egg wash on your croissants once they've finished rising. Bake the croissants in the preheated 400°F oven for 15 to 20 minutes, or until they turn a deep golden brown color on the outside. Cool slightly before serving.