Combine flour, cocoa and salt in bowl; set aside.
Combine 6 tablespoons butter and sugar in another bowl; beat at medium speed until creamy. Add egg and 3/4 teaspoon vanilla flavoring. Continue beating, scraping bowl often, until well mixed. Add melted chocolate; continue beating until smooth. Add flour mixture; beat at low speed, scraping bowl often, until well mixed.
Divide dough in half. Shape each half into ball; flatten slightly. Wrap each in plastic food wrap. Refrigerate 1 hour or until firm.
Heat oven to 350°F.
Roll out dough on lightly floured surface, one-half at a time, (keeping remaining dough refrigerated), to 1/8-inch thickness. Cut with 2-inch cookie cutter. Place 1 inch apart onto ungreased cookie sheets. Bake 5-7 minutes or just until set. Cool 1 minute on cookie sheet. Transfer to cooling rack. Cool completely.
Combine all filling ingredients in bowl; beat at low speed until well mixed. Beat at high speed until light and creamy.
Spread bottom-side of
1 cookie with about
filling; top with another cookie, bottom-side down. Repeat with remaining cookies.
Place chopped chocolate in bowl; set aside.
Heat whipping cream in 1-quart saucepan over medium-low heat just until whipping cream comes to a boil. Pour over chopped chocolate. Stir in corn syrup until mixture is smooth. Let stand at room temperature 15 minutes or until glaze is thickened.
Drizzle or spread glaze on top of each sandwich cookie. Place cookies onto waxed paper; let stand 1 hour or until glaze is set.
Store between sheets of waxed paper in container with tight-fitting lid.