Place egg whites in a large bowl; let stand at room temperature 30 minutes.
Preheat oven to 225°. In a small bowl, whisk confectioners' sugar and almond flour until blended. Beat egg whites on medium speed until foamy. Gradually add the sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Continue beating until stiff glossy peaks form. Fold in confectioners' sugar mixture.
Cut a small hole in the tip of a pastry bag or in a corner of a food-safe plastic bag; insert #805 round pastry tip. Fill bag with meringue. Pipe 1-in.-diameter cookies 1 in. apart onto parchment paper-lined baking sheets. Using a finger moistened with water, smooth tops of cookies.
Bake 40-45 minutes or until firm to the touch. Turn oven off (do not open oven door); leave cookies in oven for 1 hour. Remove from oven; cool completely on baking sheets.
For ganache, place semisweet and unsweetened chocolates in a small bowl. In a small saucepan, combine cream and corn syrup; bring just to a boil. Pour over chocolate; stir with a whisk until smooth. Remove 1 cup ganache to another bowl; refrigerate 25-30 minutes or until mixture is thick enough to pipe, stirring occasionally. Reserve remaining ganache for dipping; let stand, covered, at room temperature, stirring occasionally.
Cut a small hole in the tip of a pastry bag or in a corner of a food-safe plastic bag; insert #802 round pastry tip. Fill bag with chilled ganache. Pipe onto bottoms of half of the cookies; cover with remaining cookies. (Ganache may soften as it warms. If necessary, return ganache to refrigerator until firm enough to pipe.)
Place toppings in separate shallow bowls. Dip each sandwich cookie halfway in room temperature ganache; allow excess to drip off. (If necessary, warm ganache in microwave for 10 seconds to thin slightly.) Dip in toppings as desired; place on waxed paper and let stand until set. Store in airtight containers at room temperature.