Line an 8 x 8 inch (20×20 cm) pan with parchment paper, leaving an overhang around the edges.
In a stand mixer bowl, mix together the melted butter and brown sugar until combined. Make sure that all the butter is not separated from the brown sugar.
Add in the egg and vanilla extract and mix again.
Mix in the flour and salt. Then using a spatula, fold in the butterscotch chips.
Pour the batter into the prepared pan, and bake for 20-25 minutes until the top is just set and edges are beginning to pull away from the sides of the pan.
Let cool in the pan until completely set – at least 1 hour.
When ready to slice, lift the bars out of the pan using the overhang from the parchment paper/aluminum foil. Place on a cutting board and slice into squares with a sharp knife.