Preheat oven to 350F. Grease a 13x9x2-inch baking pan; set aside. In a small saucepan melt 1/3 cup butter; stir in 2/3 cup brown sugar, the coconut, and 3/4 cup pecans. Pat evenly in prepared pan; set aside.
In a large mixing bowl beat 1/2 cup butter with an electric mixer on medium to high speed for 30 seconds. Add 1 cup brown sugar, the baking soda, and salt; beat until combined. Beat in eggs and vanilla until combined. Add flour and beat until combined. Stir in 1/2 cup pecans and marshmallows. Spoon small mounds of mixture over coconut mixture in pan. Carefully spread to cover.
Bake about 20 minutes (mixture should be evenly browned; center may jiggle slightly when shaken). Cool in pan on wire rack. To serve, cut into bars. Drizzle with caramel topping. Makes 24 bars.