Preheat the oven to 325°F (160°C). Line baking sheets with parchment paper or silicone liners.
Whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. Add the egg, milk, and vanilla, continuing to whisk until combined.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Fold in the chocolate.
Scoop tablespoon-sized balls of dough onto the prepared baking sheet, leaving about 3 inches (7.5cm) between each cookie. Press them down slightly to flatten. If using the sea salt, sprinkle a pinch on top of each cookie.
Bake one pan at a time for 15-20 minutes, or until the edges are golden brown but the center is still soft.
Place the pans on wire racks, and allow the cookies to cool for 5 minutes before transferring the cookies from the pans to a wire rack to cool completely.
Notes
Store in an airtight container at room temperature up to a week.