Position rack in center of oven and preheat to 350° F. Line 2 baking sheets with parchment paper. Stir 1/4 C unsalted butter, granulated white sugar, and corn syrup in heavy medium saucepan over low heat until smooth and melted. Bring butter mixture to a boil over medium-high heat, stirring constantly. Remove from heat. Stir in flour. Add nuts, almond extract and vanilla; stir to combine. Drop dough by scant teaspoonfuls onto prepared baking sheets, spacing slightly more than 2" apart. Bake cookies, 1 sheet at a time, until bubbling and lightly browned, about 11 minutes. Cool on sheets 10 minutes. Transfer to rack; cool completely. Whisk remaining 1/4 cup (half stick) of butter along with powdered sugar, espresso powder dissolved in hot water, and grated chocolate together in a medium bowl to make mocha filling. Spread 1 teaspoon filling onto bottom of 1 cooled cookie. Top with second cookie, bottom side down, pressing lightly to adhere. Repeat with remaining cookies. 18-20 servings.