S'mores Thumbprints

Heat oven to 350°F. Line cookie sheets with parchment paper; set aside.

Combine butter and powdered sugar in bowl; beat at medium speed until creamy. Add egg yolk and vanilla; continue beating until well mixed. Add flour and salt; beat at low speed until well mixed.

Place egg white into bowl; lightly beat with whisk. Place graham cracker crumbs into another 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 teaspoon. Bake 9-10 minutes or until edges just begin to brown.

Immediately place 4 to 5 chocolate chips and 3 mini marshmallows in each indentation. Immediately return to oven; continue baking 3-4 minutes or until chocolate is melted and marshmallows are puffed. Remove to cooling racks. Melt remaining chocolate chips and drizzle over cookies, if desired.