1. Sift the first four ingredients together and place them in a large bowl, a food processor, or a stand mixer fitted with a paddle.
2. Add the butter cubes, and mix by hand or on low speed/short pulses until the mixture becomes sandy in texture.
3. Combine the milk and vanilla in a small cup, and add to the dough.
4. Continue kneading or mixing on low until the dough becomes a dense, cohesive mass (this shouldn't take long, and only a few pulses of the processor).
5. Turn the dough out onto a clean surface and shape it into two, 6-inch rounds. You can make the dough several days ahead through this step. Wrap it tightly in plastic, and refrigerate or freeze it. Take the dough out of the refrigerator for 10 to 20 minutes before rolling it to let it soften (longer if you froze it). 
6. Between two pieces of parchment paper, roll the dough to 1/4-inch thick (or a little thicker if you like a hefty cookie). Chill the rolled-out disc for 30 minutes.
7. Preheat the oven to 325° F and line one or two baking pans with parchment paper.
8. Dip your chosen cookie cutter in powdered sugar to prevent sticking and cut out your cookies. Coat the cutter in sugar before each new cut. Leave an inch of space between the cut cookies on the pan.
9. Sprinkle the cookies with large crystal sugar or a couple of granules of coarse sea salt, if you like.
10. Bake the cookies for 7 to 12 minutes on the center rack. The shortbread is baked when the shine has come off and the dough is just set. Cool your cookies for a couple of minutes on the baking pan, then completely on a rack. Let any traditional-style shortbread you baked in a square pan cool completely before turning out and cutting.
11. Store the cookies at room temperature in an airtight container, or freeze them.

For Chocolate Coating

1. Melt your chocolate in a small bowl set over a pot of barely simmering water. Or in 30-second bursts in the microwave, stirring frequently. 
2. Dip the baked shortbread cookies or shapes in the melted chocolate, and set them on a rack to let the chocolate harden. You can also set the cookies on a rack and drizzle the melted chocolate on top. 
3. Garnish the dipped cookies with sugar sprinkles or sea salt while the chocolate is still warm.