Preparation

• Put the butter and shortening in a 10-inch ovenproof skillet, preferably cast iron, and cook over medium heat until just melted. Remove from the heat, and then add the sugars and corn syrup, whisking gently to combine. Let rest until the butter cools slightly and is fully incorporated into the sugars, whisking occasionally, about 5 minutes.
• Add the egg and yolks one at time, whisking gently to incorporate. Whisk in the vanilla. Sift the cocoa powder over the skillet, and stir gently with a silicone spatula to combine. Sift the flour, baking soda, and salt over the skillet, and stir gently to combine. Fold in the chocolate chunks.
• Spread the dough evenly in the pan, and sprinkle with flaky sea salt. If you have the time, cover the pan with plastic wrap and let it rest in the fridge for one hour or up to 3 days.
• Position a rack in the center of the oven, and heat the oven to 350°F.
• Bake until the center looks dry and the edges are slightly cracked and just starting to come away from the sides of the pan, 20 to 22 minutes. Do not overbake; the cookie should be slightly underdone in the center. (Bake longer if you prefer a crisper cookie.)
• Serve warm, straight from the skillet, with a scoop of ice cream (or three). Or let cool slightly and slice into squares or wedges.
• Store the cookie at room temperature wrapped in plastic wrap for up to 3 days. Reheat briefly before serving.