Season duck breasts with kosher salt and freshly ground black pepper. In a saute pan or heavy cast iron pan on low heat, place breasts, skin-side down, and render the breasts until crispy and brown, about 12-15 minutes. When the breasts are crispy, transfer to a plate, meat-side down (which is still raw). Remove pan from heat, keeping fat in pan for cooking ravioli.
To make the ravioli filling: slice hot yams in half and scoop yam into a bowl. Fold in butter, syrup and five spice; check for flavor and season. To assemble ravioli: lay out a wonton wrapper, place a small mound (about 1 1/2 tablespoons) of filling in center, taking care not to get it on edges of wrapper (about 1/4-inch border). Brush the edges with egg wash and top with a second wrapper, pressing firmly to seal. Repeat with remaining wrappers and filling.
Reheat the duck fat over medium heat and sear duck breasts, meat-side down, until medium-rare, about 3-4 minutes. Let duck rest on board, meat-side down, for 5 minutes before slicing. In the same pan, fry the ravioli on both sides in the duck fat, about 1-2 minutes.
To serve, place 2 ravioli on plate, surround with duck slices and garnish with sauce.
For the Cranberry and Lemongrass Sauce:
In a medium saucepan coated lightly with oil over medium-high heat, saute shallots, garlic and lemongrass until soft, about 2 minutes. Add cranberries and saute another 2 minutes. Season with kosher salt and freshly ground black pepper to taste. Deglaze with wine, reduce by 75%. Add chicken stock and reduce by 50%. Stir in lemon juice and sugar and whisk in the butter. Check for flavor and season, if necessary.