Heat oven to 350. Line a cookie sheets with parchment paper, set aside. Combine butter and powdered sugar in large bowl, beat at med. speed until creamy. Add egg yolk and vanilla, continue beating until well mixed. Reduce speed to low, add flour and salt. Beat until well mixed. Place egg white into small bowl, lightly beat with wire whisk. Place graham cracker crumbs into another small bowl. Shape dough into 1 inch balls.(dough may be soft). Dip balls into egg white, roll in graham cracker crumbs. Reshape balls, if necessary. Place 1 inch apart onto prepared cookie sheets. Make indentation in center of each cookie with thumb or back of a teaspoon. Bake for 9-10 min. or until edges just begin to brown. Immediately place 4 or 5 choc. chips and 3 mini marshmallows in each indentation. Return to oven, continue baking for 3 to 4 min. or until choc. is melted and marshmallows are puffed. Remove to wire cooling racks. Melt remaining choc. chips and drizzle over cookies, if desired.