Preheat oven to 400°. Bring a medium pot of water to boil. Place sliced yam into boiling water, allowing to boil for 6 minutes.
Grease with butter or spray with non-stick cooking spray the sides of a 9"-10" shallow skillet.
While yams are boiling, melt butter and sugar together in the prepared skillet. Once sugar is dissolved and bubbly, add thyme leaves, shallots, cinnamon, salt, pepper and corn syrup*.
Gently drain yam slices and arrange atop the brown sugar-thyme syrup. Roll out puff pastry sheet, just enough to smooth out the seams and lay over the top of the arranged yams. Using a sharp knife, cut several ½" venting slits in the puff pastry sheet.
Place the tarte tatin into a preheated oven and bake at 400° for 35-40 minutes, or until center is golden, crisp and puffed.
Once center is cooked through, remove from oven and allow to cool 5 minutes. After the the tarte tatin has slightly cooled and sauce has slightly thickened, invert onto a serving platter.
Slice and serve.
*Corn syrup is optional, adding a slightly more gooey consistency to the brown sugar thyme sauce.