Chocolate Chip Cookie Mug Cake

1 Preheat the oven to 350°F. Lightly grease a 10- to 12-ounce oven-safe ramekin.
2 In a medium bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth.
3 Add the egg yolk and vanilla extract. Whisk until fully combined and slightly glossy.
4 Stir in the flour, baking soda, and salt until a thick cookie dough forms. If the dough seems too dry or crumbly, mix in 1 teaspoon of milk.
5 Fold in the chocolate chips, reserving a few to sprinkle over the top.
6 Press the cookie dough evenly into the prepared ramekin and top with the remaining chocolate chips.
7 Bake for 16–20 minutes, or until the edges are lightly golden and the center is just set. Be careful not to overbake—the cookie will continue to set as it cools.
8 Let cool for 5–10 minutes, then sprinkle with flaky sea salt if desired. Enjoy warm on its own or with a scoop of vanilla ice cream.