In a microwave, melt butterscotch chips and butter; stir until smooth. Cool for 10 minutes.
In a large bowl, beat the eggs, brown sugar and vanilla until blended. Beat in butterscotch mixture. Combine the flour, baking powder and salt; gradually add to batter and beat until mix well. Stir in the marshmallows, chocolate chips and nuts.
Spread into a greased 13x9-in. baking pan. Bake at 325° for 25-30 minutes or until a toothpick inserted in the center comes out clean (do not overbake). Cool completely on a wire rack.
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