To prepare cookies, combine 1 cup sugar and butter in a large bowl. Beat with a mixer at medium speed until well blended (about 2 minutes). Add 1 teaspoon vanilla and eggs; beat until combined. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, cocoa, 1 teaspoon salt, baking powder, and baking soda; stir well with a whisk. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture.
Drop dough by rounded tablespoonfuls 2 inches apart onto baking sheets coated with cooking spray. Bake at 375° for 10 minutes or until set. Cool on pans 5 minutes. Remove from pans; cool completely on wire racks.
To prepare filling, sprinkle gelatin over 1/2 cup cold water in a large bowl; set aside. Combine remaining 1/4 cup water, 1/2 cup sugar, syrup, and 1/8 teaspoon salt in a medium saucepan over medium-high heat. Cook, without stirring, until a candy thermometer registers 244°. Remove from heat. Gradually pour hot sugar syrup into softened gelatin mixture, beating with a mixer at low speed, then at high speed until thick (about 6 minutes), scraping sides of bowl occasionally. Add 1/2 teaspoon vanilla; beat until well blended.
Quickly spread about 2 tablespoons filling over bottom side of 1 cookie; top with another cookie. Repeat procedure with remaining filling and cookies.