Preheat oven to 350°F. Grease an 8 by 8-inch baking dish or pan and set aside.
Melt the butter in a large, microwave-safe bowl. Use a whisk to stir in the brown sugar until well-combined. Beat in the egg and vanilla until completely incorporated. Blend in the flour and salt until fully blended. Fold in half of the chocolate chips or other add-ins.
Scrape the batter into the prepared dish or pan, smooth the top, and sprinkle with the remaining chocolate chips/add-ins. Bake for 20 to 25 minutes or until the center is set. Set the pan on a wire rack to cool completely before slicing.
Nutrition info was calculated using semi-sweet chocolate chips as the add-in.
The baking time will be slightly shorter if you bake these blondies in a metal pan versus slightly longer if baking them in a glass baking dish.
Some people prefer gooey blondies and some people prefer them firmer. Either way, keep in mind that these are very dense, moist cookie bars that may seem a tad underdone when you first pull them out of the oven. They will continue to set and firm up as they cool. The first time you make this recipe, take note of the specific baking time for your oven that achieves your optimal blondie texture.
If you'd like to double this blondie recipe and bake it in a 9- by 13-inch pan, start checking for doneness at 25 minutes. The center pieces are always softer/gooier than the edge pieces, so a double-batch baked in a larger pan is going to result in extra center pieces. If you prefer firmer edge pieces over gooey center pieces, simply divide your doubled batter between two 8- by 8-inch pans instead of using a 9- by 13-inch pan.
These blondies will stay fresh in an air-tight container at room temperature for a few days. After that, store any leftovers in the fridge.