Butter and line an 8x8 inch baking pan with parchment paper in a way that it has two handles. Set aside.
Add dark brown sugar to a medium bowl. Add melted butter and whisk to combine until the mixture is smooth and shiny.
Make sure the mixture is not hot, otherwise it may curd the eggs.
Add vanilla extract, eggs and egg yolk. Whisk until fully combined.
Add the all purpose flour and salt. Using a spatula, fold it in and stir until combined. Don't over-mix.
Fold in the butterscotch chips or other desired mix-ins.
Transfer batter to the prepared pan.
Bake for 25-30 minutes, until the top is golden brown and it looks set and not wobbly. The sides of the cake will look done and a golden brown color. Do not over-bake, the cake will continue baking once removed from the oven, due to to the residual heat it packs.
Once done, remove pan from the oven and place it on a cooling rack. The blondies are very soft and tender now, let them cool in the pan, don't attempt to lift them right away.
Once cooled, but still slightly warm, using the parchment paper handles, lift the blondies from the pan and transfer to a cutting board.