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Preheat oven to 350°F. Line an 8×8-inch pan with parchment paper leaving a 2-inch overhang at each side.
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Combine melted butter, brown sugar, granulated sugar, vanilla, egg, egg yolk, espresso powder and salt in a large bowl and whisk vigorously to blend well until the mixture is smooth and glossy.
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Whisk in baking soda, then sprinkle the flour over the batter and fold it in. Add chocolate chunks and fold them in.
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Spread the thick batter evenly into your prepared pan. Scatter extra dark chocolate chunks on top. Bake for 25-30 minutes until the top is evenly golden brown and the edges are deeply browned. It should puff up in the center and then fall slightly, and it will still feel just slightly soft in the middle.
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Transfer pan to a wire rack, sprinkle with sea salt flakes and let cool completely before cutting into squares. These are best served the same day.
