For the Sponge: In a large nonreactive container, combine yeast, flour, and 643 grams cold water. Refrigerate overnight.
For the Dough: In a metal bowl set over a double boiler, add chocolate, cocoa powder, and butter. Once melted, transfer to the bowl of a stand mixer fitted with a paddle attachment. Mix on low speed until smooth. Set aside.
In a separate bowl of a stand mixer fitted with a paddle attachment, add milk, 980 grams ice water, Sponge, melted chocolate mixture, sugar, salt, yeast, flour, and pâte fermentée. Mix 4 minutes on low speed. Increase speed and mix additional 4 minutes, or until dough comes together.
Transfer to a work surface and divide into two 5.4-kilogram portions. Transfer to greased, parchment-lined sheet trays. Refrigerate 1½ hours.
Once chilled, to a work surface and gently press dough to degass. Transfer to an airtight container and refrigerate overnight.
For the Croissants: On a work surface, laminate each portion Dough with 1.5 kilograms butter. Once laminated, roll out until 4-millimeters thick. Cut into 3¼-inch by 6-inch rectangles.
Shape each rectangle with 1 baton dark chocolate and 2 batons milk chocolate. Transfer to sheet trays and let proof until desired size is reached.
To Assemble and Serve: Heat oven to 385°F. Bake Croissants 18 minutes. Let cool.
