Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a medium bowl, whisk together flour, baking soda, and sea salt. Set aside.
In a large bowl, combine melted butter, brown sugar, and granulated sugar. Whisk for 1-2 minutes until a smooth paste forms.
Crack in the egg and whisk until fully incorporated. Add vanilla extract and whisk until combined. Add cocoa powder. Mix to combine.
Gradually add the dry ingredients to the wet ingredients and gently fold until just combined.
Stir in chocolate chips with a spatula. Avoid overmixing.
Scoop 9 equal portions of dough onto the prepared baking sheet, leaving approximately 2 inches of space between each cookie (no more than 4 cookies per 15x10-inch baking tray).
Bake for 12 minutes or until golden and crispy at the edges and gooey in the center.
Remove from the oven and immediately sprinkle with flaky sea salt.
Allow cookies to cool completely on the baking sheet before serving.
