Into large bowl, measure flour, sugar, margarine or butter, cocoa, baking powder, and egg. With mixer at low speed, beat ingredients until well blended, occasionally scraping bowl with rubber spatula.
Preheat oven to 350°F. Grease large cookie sheet.
On lightly floured surface, with floured rolling pin, roll out half the dough into 9- by 9-inch square, about 1/8-inch thick. Cut dough square into thirds. Then cut each third crosswise into nine 3- by 1-inch rectangles. With pancake turner, place rectangles, about 1 inch apart, on cookie sheet.
With fork, prick each cookie in several places for decoration. Bake cookies 15 minutes or until firm. With pancake turner, remove cookies to wire rack to cool. Repeat with remaining dough.
While cookies are cooling, prepare mocha filling: In small bowl, with mixer at medium speed, beat 1 cup confectioners' sugar, 1/2 cup margarine or butter (1 stick), softened, 2 tablespoons unsweetened cocoa, and 2 teaspoons instant espresso-coffee granules powder until smooth.
Assemble cookie sandwiches: Spread bottom of half the cookies with mocha filling; top with remaining cookies, topside up. Store cookies in tightly covered container.
Makes about 26 cookies.