Combine flour and salt in a medium bowl; set aside. Place butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; beat until just combined. Do not whip. Add vanilla; mix. Add flour mixture; mix on low until dough starts to come together. Transfer to a floured board; knead into a ball. Wrap in plastic; chill 30 minutes.
Heat oven to 350 degrees. Roll dough on a floured surface to 1/4 inch thick. Cut out heart shapes using a 3-inch heart cookie cutter. Place hearts on an ungreased baking sheet, and bake until lightly browned, 20 to 25 minutes. Transfer to wire racks to cool.
While cookies bake, melt chocolate in a small bowl over a saucepan of gently simmering water. Add shortening; whisk until smooth. Dip cooled shortbread into chocolate to coat surface evenly, or brush on with pastry brush; let set. Color remaining chocolate mixture, and pipe onto cookies as desired.