Preheat oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large bowl, whisk melted butter and brown sugar until smooth.
Beat in eggs and vanilla extract until fully combined.
In another bowl, whisk flour, baking powder, and salt together.
Gradually mix dry ingredients into wet ingredients until just combined.
Fold in butterscotch chips evenly.
Spread batter into the prepared pan and smooth the top.
Bake for 25–30 minutes or until a toothpick inserted comes out clean.
Let cool completely before cutting into squares.