Preheat oven to 350. Line an 8-inch square pan with nonstick foil or parchment.
Gently mix together the melted butter, brown sugar, egg, egg yolk and vanilla.
Mix the flour, baking powder, baking soda and salt together in a separate bowl, then stir them into the sugar mixture. Stir in butterscotch morsels (aka chips).
Pour into pan, drop on counter to remove air bubbles. Bake for 25-27 minutes on center rack. Cool in pan set on a wire rack.