Use a fork to poke holes into 1 sweet potato. Wrap the sweet potato in a damp paper towel and microwave on high for 10-12 minutes, until fork-tender.
Peel and dice the other sweet potato into 1-centimeter (½ inch) cubes.
Carefully scoop the pulp of the cooked sweet potato into a food processor or blender. Add about 1 teaspoon each of salt and pepper and blend under smooth. Add a splash of water if the puree is too thick.
In a pan over medium heat, add 1 tablespoon olive oil and the mushrooms. Cook until the mushrooms start to crisp, about 3 minutes.
Add 1 teaspoon each salt and pepper to the mushrooms and remove from the heat. Drain the mushrooms of excess oil in a paper towel-lined bowl.
Add 6 tablespoons of the butter and sage to the pan and cook until brown bits start to form on the bottom, being very careful to not burn the butter.
Once slightly brown, remove the sage and add the cubed sweet potato and 1 teaspoon each of salt and pepper. Cook about 5 minutes, until the potato becomes tender.
Stir in the reserved purée and chicken stock. Simmer for two minutes, until the sauce thickens slightly and the mixture comes together smoothly.
Add the penne and mushrooms and coat evenly in the sauce.
Finish the pasta with 1 tablespoon butter, 1 tablespoon of olive oil, and sherry wine vinegar, stirring to combine. Remove from heat and stir in the Parmesan cheese.
Serve warm, garnished with chives, crushed amaretti cookies, and more Parmesan.