Preheat oven to 350 degrees. Spray an 8-inch square baking pan with nonstick baking spray with flour. Set aside.
In a medium bowl, whisk together melted butter and brown sugar until smooth. Whisk in eggs and vanilla
In another medium bowl, whisk together flour, baking powder and salt. Gradually add egg mixture to flour mixture, stirring to combine. Gently stir in butterscotch and walnuts. Spread batter in prepared pan, smoothing top with an offset spatula.
Bake until golden brown and a wooden pick inserted in center comes out clean, approximately 20 minutes. Let cool completely in pan on wire rack.