Preheat oven to 350F degrees, place oven rack at middle position and line two baking sheets with parchment paper. Set aside.
Melt butter in microwave for 40 seconds until barely melted. You want the butter melted but not hot.
In large bowl, using hand mixer, or in bowl of stand mixer, combine melted butter, granulated sugar, and brown sugar. Mix on low until creamy. Add the vanilla and mix until incorporated.
Add the egg and mix ONLY for 10-15 seconds, still on low speed. This is important as overbeating the mixture with the egg will make for a too-stiff batter. Turn off the stand mixer or beaters.
In small bowl or parchment envelope, combine flour, baking soda and salt.
Add about 1/3 flour mixture and mix on low for just a few seconds, the flour won't be completely mixed in. Continue to add flour mixture until fully incorporated and beat, still on low until completely mixed. Your cookie dough, at this point, should hold together easily when pressed together, but not be too wet. If it seems too wet, feel free to add another tablespoon of flour at a time until it resembles photos (I've never had to do this, though).
Add 1/2 cup semi-sweet chocolate chips and mix on low about 5 seconds, just until mixed throughout. If you like a lot of chips in your cookies, add an additional 1/4 cup chips.
Cook's note: You can also use a combination of chocolate chips and chocolate chunks. If you like really sweet cookies, feel free to use milk chocolate chips. I've experimented with milk chocolate, semi-sweet, and bitter chocolate. Semi-sweet seems to please everyone.
Using a 2" cookie scoop, loosely scoop dough and place 2 1/2" apart (6 cookies per baking sheet). Place one cookie sheet in the oven on middle rack and bake for 9-11 minutes until cookies are poofed and still white. They will not look done, you might barely see golden color around edges of some cookies. (I've tested these over and over and with my oven, 10 minutes is exactly perfect. The cookies become completely poofed and white at about 9 1/2 minutes, an additional 30-45 seconds ensures additional cooking inside while still being soft when cooled.)
Remove cookies from oven and leave on baking sheet for additional 15-30 minutes. While one baking sheet is cooling, bake the remaining 6 cookies on the second baking sheet in same manner.
While the cookies cool, they will kind of sink and darken on the baking sheet. After 15-30 minutes, they're set and can be served or stored in an air-tight container for up to 3 days. The cookies are absolutely perfect about 4 hours after cooking. During this time, the flavors meld perfectly and the softness settles and becomes less "wet" and more chewy.
FYI- I've made this same cookie recipe, substituting white chocolate chips and macadamia nuts. We think they're even better than the semi-sweet chocolate cookies! For this alteration, I use 1/2 cup good-quality (like Ghiradelli) white chocolate chips, and then I use about 3/4 whole macadamia nuts (pre-chopped are chopped a little too finely for my taste), and then rough chop them, leaving some in half for a good nutty bite.