Preheat your oven to 180°C (350°F). Line a baking sheet with parchment paper and gather your ingredients.
Melt the butter in a saucepan over low heat until just warm and glossy, then remove from heat and let it cool slightly.
In a mixing bowl, whisk together the brown sugar and cocoa powder until well combined and slightly fragrant, about 1 minute.
Add the large egg and vanilla extract to the sugar mixture. Beat until smooth and slightly frothy, about 30 seconds.
Pour in the cooled melted butter and stir until fully incorporated, creating a glossy, fudgy batter.
Sift together the cocoa powder, baking soda, and salt directly into the wet ingredients. Mix gently until just combined.
Fold in the all-purpose flour until the dough is thick and fudgy, just until no streaks remain.
Stir in the dark chocolate chunks, dispersing them evenly throughout the dough for melty pockets in each bite.
Using a cookie scoop or tablespoon, portion dough onto the prepared baking sheet, spacing about 2 inches apart to allow for spreading.
Bake in the preheated oven for 10-12 minutes, until the edges are set and crackly, but the centers still look soft and fudgy.
Remove the cookies from the oven and let them rest on the baking sheet for 5 minutes to firm up slightly, then transfer to a wire rack to cool completely.
Enjoy these cookies at room temperature, reveling in their crackled crust and gooey, brownie-like interior.
