When the cream is hot, pour it over the white chocolate and butter mixture and whisk to combine. Allow to cool to room temperature before covering and refrigerating for at least 4 hours, or until completely chilled.

In a medium bowl, whisk together 1 3/4 cups plus 1 1/2 tablespoons all-purpose flour, 1 cup plus 1 1/2 tablespoons cocoa powder, and 3/8 teaspoon baking soda. Set aside.

Mound the dough on a work surface lined with wax paper and push it together until it forms a 6-inch square block. Wrap in plastic wrap and refrigerate for at least an hour, or until firm.

When the dough is chilled and you're ready to bake, position a rack in the center of the oven and preheat the oven to 325 (F). Prepare a pan by lining it with parchment paper or a Silpat mat.

Unwrap the dough and place it between two pieces of wax paper. With a rolling pin, pound the top of the dough, working from left to right, and begin to flatten it. Turn the dough 90 degrees and repeat — this will help prevent the dough from cracking as it is rolled. Roll out to a 1/8-inch thick sheet. Be careful not to overwork the dough and let it thaw too much, otherwise it'll be too hard to work with! If it gets too melty, simply refrigerate it until it's firm enough to cut.