Step 1: Preheat your oven to 350°F (175°C) for even baking.
Step 2: Melt the 6 tablespoons of salted butter in a medium mixing bowl. Allow it to cool slightly.
Step 3: Add the 2 tablespoons of granulated sugar and 2 tablespoons of brown sugar to the melted butter. Whisk until smooth.
Step 4: Stir in the 1 teaspoon of vanilla extract and optional 1/4-1/2 teaspoon of instant espresso powder.
Step 5: Add the large egg and whisk until fully incorporated.
Step 6: Sift in the 1/3 cup plus 1 tablespoon of all-purpose flour and 1/4 teaspoon of baking powder. Stir gently to combine.
Step 7: Fold in the 1/2 cup of dark chocolate chunks.
Step 8: Pour the mixture into your prepared baking dish and bake for 20-25 minutes until edges are set.
Step 9: Sprinkle flaky sea salt on top after removing from the oven.
Step 10: Let cool for 10 minutes before slicing into squares and serving warm.
