Two flavors of dough are formed into a single log, then sliced and baked in these tree ring-like cookies. Martha gilds the lily with milk chocolate "bark" and pistachio "moss" for a unique faux-bois effect.
For simpler shortbread swirls without the faux-bois effect, after slicing dough 1/4 inch thick, transfer to prepared sheets, spaced about 1 inch apart. Bake until firm to the touch and just lightly golden on the bottom, about 18 minutes. Transfer sheets to wire racks and let cool completely.