Heat oven to 375°F. Mix flour, cocoa powder, baking powder and salt in medium bowl.
Beat butter, sugars in bowl until creamy. Beat in eggs, one at a time. Add vanilla. On low, beat in flour mixture.
On floured surface, roll out half of dough to 1/8-inch thickness. Cut with 1-3/4-inch heart-shaped or small round cookie cutter. Transfer to ungreased baking sheet.
Bake in 375°F oven 8 to 10 minutes. Transfer cookies to rack to cool. Roll, cut and bake remaining dough.
Beat sugar, shortening, rind and juice in bowl until smooth but thick. Spread 1 teaspoon on half the cookies. Sandwich with second cookie. Let set 30 minutes.