Triple-Chocolate-Mint Sandwich Cookies

In a medium bowl, combine the granulated sugar, oil, melted white chocolate, egg whites, mint flavoring, and butter flavoring, stirring until well mixed. In a small bowl, stir together flour, baking powder, and salt. Add the flour mixture to the egg white mixture; stir just until combined. Cover and chill dough for at least 1 hour or up to 24 hours or until firm enough to roll into balls.

Preheat oven to 350 degrees F. Shape dough into 3/4-inch balls (1 teaspoon dough each). Place cocoa powder in a small bowl. Roll balls in cocoa powder to coat. Place balls 1-1/2 inches apart on ungreased cookie sheets. Flatten balls with the bottom of a glass to about 1-1/4-inch-diameter circles.

Bake for 6 to 7 minutes or just until edges are firm. Transfer to a wire rack; cool.

To prepare the chocolate filling: In a small bowl whisk together the powdered sugar, the melted bittersweet chocolate, and enough of the water to make a smooth spreadable filling. (If mixture seems too thin, allow it to stand a few minutes before using.)

Spread the bottoms of half of the cookies with the chocolate filling, using about 1 teaspoon filling on each. Top with the remaining cookies, bottom sides down, to make sandwich cookies.

If desired, place the 1 ounce melted white chocolate in a small resealable plastic bag. Seal bag. Using scissors, snip off a very small corner from bag. Drizzle white chocolate over tops of cookies; place mint leaves on top.