In a microwave, melt butter and chocolate; stir until smooth. Cool slightly. In a large bowl, beat the eggs, sugar and espresso powder until thickened, about 5 minutes. Stir in vanilla and chocolate mixture. Combine flour and salt; gradually add to chocolate mixture. Fold in walnuts and chocolate chips.
Transfer to a greased 13x9-in. baking pan . Bake at 350° for 25-30 minutes or until a toothpick inserted in the center comes out with moist crumbs (do not overbake).
For topping, in a large bowl, combine the marshmallows, walnuts and chocolate chips. Sprinkle over top. Bake 4-6 minutes longer or until marshmallows are lightly browned. Cool on a wire rack. Cut into bars.