Step 1

Arrange a rack in the middle of the oven and heat the oven to 250ºF.

Step 2

Coarsely chop about 8 ounces white chocolate and place in a quarter (9x13-inch) rimmed baking sheet or metal baking dish of similar size.

Step 3

Bake for about 1 hour. Every 10 to 15 minutes, give the chocolate a thorough stir, scraping all the chocolate up from the bottom to prevent it from burning -- sometimes the chocolate will clump and seem grainy. Once the chocolate has turned to a dulce de leche-like caramel colour, give it a very vigorous stir until smooth and glossy. Scrape it into a wide, shallow plastic container, preferably something larger than 6 inches wide. Refrigerate uncovered until solid.

Step 4

Place 2 sticks unsalted butter in the bowl of a stand mixer (or a large bowl if using an electric hand mixer) and let sit at room temperature until softened. Take the caramelized white chocolate out of the container and chop into 1/4-inch pieces.

Step 5

Sift 1 cup plus 2 tablespoons all-purpose flour, 1 1/3 cups rye flour, 1/2 cup black cocoa powder, and 3/4 teaspoon baking soda into a large bowl. Push any lumps of cocoa powder through the sifter. (If there are little flakes of bran left in the sifter from the rye flour, you can add to the flour mixture or leave them out.) Add 1/2 teaspoon flaky salt and whisk everything together.

Step 6

Add 1/2 cup plus 2 tablespoons granulated sugar and 1/2 cup plus 1 tablespoon packed light brown sugar to the butter. Beat with the paddle attachment on medium speed just until smooth and creamy, 2 to 3 minutes. Add 1 teaspoon vanilla extract and beat briefly to combine.

Step 7

Turn the mixer off and pour in the flour mixture. We don’t want to make a huge mess and have a cloud of flour and cocoa powder cover every surface in your kitchen, so cover the bowl of the mixer with a kitchen towel and then gently pulse the mixer just to get the flour combined a little before turning the mixer to low to fully mix it in properly. We want to mix in the flour just until it disappears into the butter but before the mixture forms a ball of dough. It should still look a little crumbly.

Step 8

Add the caramelised white chocolate and mix briefly just to combine. Transfer the dough to a work surface and use your hands to form it into a uniform dough. Cut the dough into two pieces. Form each piece into a log that’s about 2 inches wide. Wrap each log in parchment paper or plastic wrap. Refrigerate until firm, 3 to 4 hours.

Step 9

Arrange 2 racks to divide the oven into thirds and heat the oven to 325ºF. Line 2 baking sheets with parchment paper.

Step 10

Unwrap one of the logs of dough and use a sharp, thin knife to cut into slices about 1/2-inch thick. If the slices fall apart, just press them back together. Place on the baking sheets, evenly spacing them out. The cookies will spread but not significantly. Sprinkle with a little extra flaky salt.

Step 11

Bake until set around the edges and but still a little soft in the middle, about 14 minutes. Let cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely. The cookies will crisp up, leaving you with a wonderful sablé texture.