In a microwave, melt butterscotch chips and butter at 70% power for 1 minute; stir. Microwave at additional 10- to 20-second intervals, stirring until smooth. Cool slightly.
In a large bowl, beat the eggs, vanilla and butterscotch mixture. Combine flour, brown sugar, baking powder and salt; gradually add to butterscotch mixture (batter will be thick). Fold in marshmallows, chocolate chips and walnuts.
Spread into a well-greased 15x10x1-in. baking pan. Bake at 350° for 20-25 minutes or until a toothpick inserted in the center comes out clean (do not overbake). Cool on a wire rack. Cut into bars.