Preheat oven to 350° Fahrenheit and line three baking sheets with parchment paper.
In a medium size bowl, cream butter, sugar, and brown sugar together with an electric mixer on medium speed for 1-2 minutes, until smooth and light colored, but don't whip.
Add egg and vanilla extract and continue mixing for another minute, just until combined.
In a separate bowl, whisk flour, baking soda, and sea salt together.
Add the flour mixture to wet ingredients, mixing with a large spoon or rubber spatula until just combined, but still dry and crumbly (don't fully mix the flour in at this point, since we're going to stir the dough again when we add the mix-in).
Fold in 1/2 to 3/4 cup of your mix-in (chips, chopped chocolate or candy, dried fruit, and/or nuts).
Using a 1.5 TBS cookie scoop, scoop the dough onto lined baking sheets a couple of inches apart to allow for some spreading.
Bake the cookies for 9-10 minutes, rotating the pan halfway through to ensure even baking.
Let the cookies cool on the cookie sheet for a couple of minutes before transferring to a wire rack to cool completely. They will be very soft when they come out of the oven.
Store the cookies in airtight container for up to 5 days or freeze for up to 3 months.
Notes
Use softened butter, but not too warm or the cookies will spread.
Don’t overcream the dough, mix just until light and combined.
Spoon and level flour to avoid dry cookies.
Don’t overmix the dough once flour is added.
Use dark brown sugar for a softer, chewier cookie texture.
Add anywhere from 1/2 cup to 3/4 up mix-ins.
Cookies should look slightly underdone in the center when you pull them out of the oven.
Let the cookies cool on the pan for a few minutes before moving to a wire rack, they will be very soft just out of the oven.
The nutrition facts are for the cookie dough only. It does not include any mix-ins, since they will vary.