The Bojon Gourmet

Prepare the detrempe:
1 Heat the milk over a low flame until it is just warm to the touch (110 - 120ºF), and place in the bowl of a stand mixer fitted with the dough hook. Sprinkle the yeast over the milk and let stand 10 minutes. Add the 3/4 cup rye flour, 2 cups of the bread flour, and the sugar and salt to the bowl. Mix the dough on medium-low speed for 7 minutes, adding more bread flour by the tablespoon within the first 2 minutes until the dough comes away from the sides of the bowl but still feels slightly sticky to the touch. Add the softened butter a little at a time and mix until completely incorporated. Scrape the dough into a medium-sized bowl, cover tightly, and chill for 1 hour. This dough is called the detrempe. (No need to wash the mixer just yet.)
Prepare the butter block:
1 Meanwhile, place the 3 sticks of cold butter cubes in the bowl of the stand mixer and beat with the paddle attachment on medium-high speed until the butter is smooth, but still cool. Scrape the butter onto a piece of plastic wrap and shape it into a fairly thin 6" square. Chill the butter block 30 minutes.
Laminate the dough:
1 Scrape the chilled detrempe onto a surface dusted lightly with rye flour and shape it into a square. Use the blades of your hands to press a diamond shape into the center of the detrempe leaving 4 triangular wings. Roll out the wings into flaps and flatten the center diamond slightly. Place the butter square atop the diamond, and fold the flaps up over the butter to enclose it.
2 Gently begin rolling the dough into a large rectangle, dusting the rolling pin and surface with just enough rye flour to keep the dough from sticking, and brushing off the excess flour with a pastry brush. When the dough is thin enough that you can see the butter through the dough (but not so thin that it breaks and exposes the butter), fold the rectangle in thirds, like folding a letter. Starting with a short end, roll the dough into a loose spiral. Wrap the dough and chill 1 hour to relax the dough and firm up the butter. Repeat the rolling and folding process once more. Wrap the dough and chill at least 1 hour and up to 1 day.
Shape the pastries:
1 Roll the dough out into a 12x18" rectangle, dusting the surface with rye flour as needed and sweeping away the excess with a pastry brush. Trim away the outer inch or so of dough to form an even rectangle. (The scraps can be tied into loose knots, brushed with egg wash, dusted with cinnamon sugar and baked.) Use a pizza wheel or sharp chef's knife to cut the dough in half the long way, then cut each half crosswise into six smaller rectangles. Place about 3/4 ounce of chocolate down the center of each rectangle, the short way, and fold up each end so that the edges overlap loosely. Repeat with the remaining rectangles. Place the pastries seam side-down on two rimmed baking sheets lined with parchment paper (for easy clean-up).
2 Slip the whole baking sheet into a large, clean, plastic trash bag, inflate the bag so that it isn't touching the croissants, and secure it shut with a clothespin. (Alternately, you can cover the croissants loosely with plastic wrap sprayed with non-stick spray.) Let the pastries rise in a warm-ish spot until they're puffy, 1 - 1 1/2 hours; they won't double in bulk. If you poke one gently, it's ready to bake if the depression left by your finger doesn't bounce back.
3 Meanwhile, position racks in the upper and lower thirds of the oven and preheat to 400ºF.
Bake the pains au chocolat:
1 Unwrap the pastries and brush them with the beaten egg. Bake the pastries until deep golden brown, 18-22 minutes, rotating the pans once toward the end of the baking time. Let the pastries cool for at least 10 minutes. Pains au chocolat are best when warm, when the chocolate is still soft, but extras will keep at room temperature for up to a few days. Rewarm in an oven or toaster oven for yummiest results. Baked pastries can also be frozen and "refreshed" in a hot oven when ready to eat.