Whisk the melted butter and sugars in a large bowl until combined.
6 tablespoons butter, ½ cup brown sugar, ¼ cup granulated sugar
Add the egg and vanilla and mix well.
1 large egg, 1 teaspoon vanilla extract
Using a rubber spatula, fold the flour, salt, and baking soda into the butter mixture until just combined; do not overmix.
1 ⅛ cups all-purpose flour, ½ teaspoon kosher salt, ¼ teaspoon baking soda
Fold in the chocolate chips.
¾ cup semisweet 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.