Preheat the oven to 350°F (175°C): Lightly butter and flour an 8x8-inch (20 cm x 20 cm) pan.
Make the batter: Whisk together the melted butter and sugar in a bowl. Add the egg and vanilla extract and whisk to combine. Add the flour, baking soda, baking powder, and salt, and mix it all together. Add the butterscotch chips or other mix-ins (up to 1 cup total).
Bake the blondies: Pour into the pan and spread evenly. Bake for 18 to 25 minutes or until a tester inserted into the center comes out clean.
Cool and serve: Allow to cool. Cut into squares or rectangles and serve. Store blondies in an airtight container at room temperature for up to 1 week, or freeze for up to 3 months. Did you love the recipe? Give us some stars and leave a comment below!