Heat oven to 350°. Prepare two cookie sheets by lining with parchment paper, silpat liner, or spray lightly with cooking spray. I prefer parchment paper.
Unwrap butter and place in a microwave-safe bowl. Microwave for 42 seconds. Butter should be half melted/half soft chunk.
In a mixing bowl, or use a stand mixer with the paddle attachment, add the microwaved butter, granulated sugar, and brown sugar. Beat until creamy, fluffy looking, and pale in color. About 1-2 minutes.
Add the egg and vanilla extract. Beat just until combined, no longer than 15 seconds. You don't want to over beat the egg into the cookie dough.
Add the flour, baking soda, and salt. Beat together on low speed just until combined. At this point you can take clean hands and feel the dough, it should be super soft and form a soft ball.
Add the milk chocolate chips and mix together with your hands. Or you can use a wooden spoon or spatula of some sort. I love using my hands because then you just barely mix in the chocolate chips without over mixing, and you can make sure the dough is super soft and the right texture.
Take about 2-3 tablespoons of dough and roll into a ball. The dough balls should look like the size of a golf ball. Place 9 cookies on the first sheet and 6 cookies on the other cookie sheet.
Bake one cookie sheet at a time for 8-10 minutes.
I have never baked my cookies for more than 8 minutes. The cookies should look very pale and puffy. You don't want the cookies to have any golden brown color on them. Take them out of the oven and let the cookies cool on the cookie sheet for 30 minutes.
Leftovers can be stored in an airtight & covered container at room temperature for several days.