Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, whisk together the melted butter, dark brown sugar, granulated sugar, vanilla extract, egg, egg yolk, espresso powder, and salt until smooth.

Gently mix in the baking soda, then add the flour and fold until no dry streaks remain. Incorporate the chopped dark chocolate.

Transfer the batter to the prepared baking pan and spread it evenly. Optionally, sprinkle extra chocolate chunks on top.

Bake for 25-30 minutes until the top is golden-brown and slightly soft in the center.

Let the blondies cool in the pan for 10 minutes, then transfer to a cooling rack, sprinkling with flaky sea salt before cooling completely.