Preparation Steps

• Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat and set aside.
• In a medium mixing bowl, cream the softened butter and packed brown sugar together until smooth and slightly fluffy — about 1 minute using a hand mixer or 2–3 minutes by hand with a whisk.
• Whisk the egg yolk and vanilla into the butter-sugar mixture until fully incorporated and glossy.
• In a separate bowl, sift (or whisk) the cocoa powder with the flour, instant espresso powder, baking soda, salt, and ground cinnamon until evenly combined and free of lumps.
• Fold the dry ingredients into the wet mixture in two additions, mixing just until no dry streaks remain. Stir in the chocolate chunks by hand so they stay in distinct pockets throughout the dough.
• Scoop the dough into nine equal portions (about 2 tablespoons each) and roll into balls. Arrange on a plate or tray, cover loosely, and chill in the refrigerator for 15 minutes to firm up — this yields thicker, fudgier cookies.
• Place the chilled dough balls on the prepared sheet about 2 inches apart. Press an extra chocolate chunk into the top of each ball for a pretty finish.
• Bake in the center of the oven for 11 minutes, or until the cookie tops look set and the edges are just firm but centers still appear slightly soft. Ovens vary; if you prefer chewier cookies, err on the shorter side.
• Allow the cookies to cool on the baking sheet for 4–5 minutes so they set, then transfer to a wire rack to cool slightly more. If desired, sprinkle a tiny pinch of flaky sea salt over the warm cookies before serving.