Whisk the melted butter and sugars in a large bowl until combined.
Add the egg and vanilla and mix well.
Using a rubber spatula, fold the flour, salt, and baking soda into the butter mixture until just combined; do not overmix.
Fold in the chocolate chips.
Pour the batter into the prepared pan, smoothing the top with the spatula.
Bake until the top of the bars is light golden brown, slightly firm to the touch, and edges start pulling away from sides of pan, about 15-20 minutes.
Cool on a wire rack to room temperature.
Video
Notes
To melt the butter, I microwave it until it is melted about 90% of the way. Then, I whisk it until it is completely melted. Butter that's too hot can give you greasy cookie bars.
I recommend mixing this dough by hand. It's easy to do, and it helps to prevent overworking the dough.