Step 1

Melt 3 sticks unsalted butter in a large, light-colored skillet over medium heat. Continue to cook, stirring occasionally, until the milk solids have separated from the fat and turn dark golden brown and smell nutty, 8 to 10 minutes more. Carefully scrape the browned butter into the bowl of a stand mixer (or large heatproof bowl if using an electric hand mixer). Refrigerate until firm and cold to the touch, 1 1/2 to 2 hours.

Step 2

Place 2 1/4 cups all-purpose flour, 2 tablespoons spelt flour, 2 teaspoons baking soda, and 1 teaspoon kosher salt in a medium bowl and whisk to combine. Chop 20 ounces dark chocolate into pieces no larger than 1/4 inch if needed (about 4 cups).

Step 3

Add 1 cup plus 1 1/2 teaspoons granulated sugar and 1/4 cup plus 2 tablespoons packed brown sugar to the browned butter. Beat with the paddle attachment on medium speed until lightened in color and fluffy, about 3 minutes. Beat in 2 large eggs, one at a time, waiting until the first is completely incorporated before adding the next. Stop and scrape down the sides of the bowl with a flexible spatula.

Step 4

Add 1/4 cup sourdough starter and 1 tablespoon plus 1 1/2 teaspoons vanilla extract and beat on medium speed until combined and smooth, about 1 minute.

Step 5

Add half of the flour mixture and beat on the lowest speed until just combined. Add the remaining flour mixture and beat until just combined. Add half of the chocolate chunks (about 2 cups) and beat until just combined. Cover the bowl and refrigerate until the dough is firm enough to scoop, 1 to 2 hours.

Step 6

Place the remaining chocolate chunks in a medium bowl. Scoop the dough out in 1/4-cup portions (50 grams each). Form into balls, then roll in the chocolate chunks so some of them adhere. Place in a single layer on a baking sheet. Cover and refrigerate overnight or for up to 2 days.

Step 7

Arrange 2 racks to divide the oven into thirds and heat the oven to 350ºF. Line 2 baking sheets with parchment paper or silicone baking mats.

Step 8

Transfer the dough balls to the baking sheets, spacing them at least 3 inches apart, 8 per baking sheet. Bake for 7 minutes. Rotate the baking sheets between racks and from front to back. Bake until the edges are starting to brown and the centers are set, 6 to 7 minutes more.

Step 9

Sprinkle with flaky salt if desired. Let cool on the baking sheets for 10 minutes. Transfer to wire racks and let cool completely. Repeat baking the remaining dough balls on cooled baking sheets, you can reuse the parchment or baking mats.