Preheat oven to 350 degrees. Line a buttered 8-inch square baking pan with foil or parchment paper, allowing a 2-inch overhang. Butter lining (excluding overhang); set pan aside.
Whisk together flour, baking powder, and salt in a medium bowl; set aside.
Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy, about 3 minutes. Add eggs and vanilla; mix until combined. Reduce speed to low. Add flour mixture, and mix, scraping down sides of bowl, until well combined. Mix in butterscotch, cashews, and toffee.
Pour batter into prepared pan; spread evenly with a rubber spatula. Bake until golden brown and a cake tester inserted into blondies (avoid center and edges) comes out with a few crumbs but is not wet, 42 to 45 minutes. Let cool slightly in pan, about 15 minutes. Lift out; let cool completely on a wire rack before cutting into squares. Blondies can be stored in an airtight container at room temperature up to 3 days.