Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking soda, and sea salt until well combined. Set aside.
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer, beating on medium speed until the mixture is light and fluffy, about 3-4 minutes.
Add the eggs one at a time to the butter-sugar mixture, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until combined. Be careful not to overmix.
Fold in the semi-sweet and milk chocolate chips, and walnuts if using, until evenly distributed in the dough.
Using a large cookie scoop or measuring cup, portion out the dough and place the dough balls on the prepared baking sheet, leaving enough space between each cookie (about 3 inches apart).
Bake the cookies in the preheated oven for 12-15 minutes, or until the edges are golden brown but the centers are still soft. They will continue to cook as they cool on the baking sheet.
Once baked, remove from the oven and let the cookies rest on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.
Notes
Serve warm with a glass of milk for the ultimate indulgence.