Preheat oven to 350 degrees F. Grease a 13x9x2-inch baking pan; set aside. In a small saucepan heat the 1/3 cup butter over medium heat until melted. Stir in coconut, the 3/4 cup pecans, and the 2/3 cup brown sugar. Press mixture evenly onto bottom of prepared pan; set aside.
In a large bowl beat the 1/2 cup butter with an electric mixer on medium to high speed for 30 seconds. Add the 1 cup brown sugar, the baking soda, and salt. Beat until combined, scraping side of bowl occasionally. Add eggs and vanilla; beat until combined. Beat in flour until combined. Stir in the 1/2 cup pecans and the marshmallows. Carefully spread mixture over coconut layer.
Bake about 20 minutes or until top is evenly browned (center may jiggle slightly when gently shaken). Cool in pan on a wire rack. Cut into bars. If desired, drizzle with caramel topping and garnish with pecan halves.