1-1/2 cups (350 milliliters) roughly chopped dark chocolate
120 grams piece dark chocolate
Sea salt flakes, for garnish
Preheat oven to 325 F (160 C).
In a large bowl, mix together the flour, salt, sugar, and yeast until combined. Cut the butter into the flour mixture using a pastry cutter and work in until the mixture starts to come together. Add water and milk and knead on a lightly floured surface until the mixture forms a dough and is somewhat smooth, but the butter is not yet fully incorporated. Form the dough into a ball, then cover in plastic wrap and refrigerate for 20 minutes.
Roll the dough out onto a lightly-floured surface into a large rectangle. Brush the surface evenly with the melted butter. Fold the bottom third of the rectangle up toward the centre of the dough, then fold the top third of the rectangle down to meet it. Cover in plastic wrap and refrigerate for 20 minutes.
Remove the plastic wrap and repeat the process of rolling out into a rectangle and folding into thirds, then return to the fridge for another 20 minutes. Repeat the entire process of folding and refrigerating one more time.
Roll the dough out into a 20.5 centimeter wide rectangle that is roughly eight millimeters thick. Cut rectangle evenly into five smaller rectangles. Divide the chopped chocolate along the five rolls, placing it along one of the shorter edges (but leaving a small border), then grate a light dusting of chocolate from the solid piece of chocolate over the rest of the pastry, leaving a small border on both ends. Roll up the dough around the chopped chocolate to make a roll. Place seam side-down on a parchment paper-lined baking sheet, then use your palm to press down and slightly flatten the roll.
Whisk together eggs and milk to make an egg wash, the brush over the tops of each roll. Sprinkle over with a little sea salt, then place in the oven to bake until the pastry is golden brown and crisp, about 40 minutes.