Make the cookies: Combine the flour and salt in a bowl. Put the butter, granulated sugar and confectioners' sugar in the bowl of a stand mixer fitted with the paddle attachment; beat on medium speed until light and fluffy, about 4 minutes (or about 6 minutes if using a hand mixer). Add the egg yolks one at a time, beating on medium-high speed between each addition. Add the vanilla and lemon zest and beat until combined. Add the flour mixture in 2 batches and mix until just incorporated. Divide the dough in half, wrap in plastic wrap and chill at least 1 hour.
Line 2 baking sheets with parchment and lightly oil. Roll out each piece of dough between 2 sheets of parchment until 1/8 inch thick (return the dough to the refrigerator if it softens too much). Cut into 2-to-4-inch shapes with cookie cutters; arrange 2 inches apart on the prepared baking sheets. Reroll the scraps and cut out more cookies. Sprinkle with sanding sugar. (To make a design like the one above, use a potato masher, slotted spoon or spatula as a stencil.) Chill the cutouts 1 hour.
Preheat the oven to 350 degrees F. Bake the cookies until lightly golden around the edges, 12 to 15 minutes. Let cool 3 minutes on the baking sheets, then transfer to racks to cool completely.
Meanwhile, make the icing: Put the chocolate, shortening, corn syrup, milk, confectioners' sugar and vanilla in a microwave-safe bowl. Cover with plastic wrap and microwave on high in 30-second intervals, stirring, until smooth. Tint with food coloring.
Ice the cookies as desired and decorate with toppings. Transfer to the rack and let set.
Thumbprints are traditionally filled with jam, but these are much more fun: Try chocolate kisses, crushed candy canes or a dollop of frosting in the center.