Preheat the oven to 350°F and line an 8×8-inch baking pan with parchment paper, letting the parchment rise up over the side for easy removal after baking. Set aside. In a large microwave-safe bowl, melt the butter then whisk in the brown sugar and vanilla until smooth. When this mixture is sufficiently cooled, whisk in the egg.
The bars need to be stored in an airtight container in the refrigerator. You can then enjoy them for up to 5 days. Or you can freeze them for up to 3 months! Before enjoying them from frozen, you’ll need to place them in the refrigerator to thaw overnight.