Microwave: Pierce sweet potatoes all over with a fork, then wrap in damp paper towels. Microwave on high for 7 to 10 minutes, until fork-tender. Allow potatoes to cool enough to touch, then cut in half and scoop out the flesh, discarding the skin.
Mash: Measure out 3 cups of the puree and allow it to cool to room temperature. Puree sweet potatoes using either a masher or potato ricer. Transfer to a large bowl and stir in the egg.
Dough: Stir salt into 1 ½ cups of flour. Sprinkle flour mixture over the sweet potato and fold to combine (do not over stir - this will lead to gluey gnocchi). Add more flour as needed until you reach a sticky dough that can be formed into a loose ball (the goal is to use as little flour as possible).
Roll: Transfer dough onto a well-floured surface. Portion dough into fist-sized balls and roll each into 1 inch thick ropes. Cut the ropes into about ½ inch pieces. For the classic gnocchi shape (optional), slide each gnocchi over the tines of an upside down fork using your thumb to press it down the fork*.
Boil: Bring a large pot of water to a boil. Working in batches, drop each gnocchi into the boiling water. When the gnocchi floats to the top it's done (2 to 3 minutes). Remove with a slotted spoon and set on a paper towel.
Saute: Melt butter in a large saucepan over medium heat, then add garlic, sage, rosemary, and gnocchi. Cook until gnocchi is lightly browned, about 5 minutes (you may need to work in batches to cook all the gnocchi).
Serve: Remove pan from heat and transfer all gnocchi back into the pan. Top with parmesan cheese and drizzle with cream, stirring to coat. Bon apetit!
*At this point you can freeze your sweet potato gnocchi! Freeze in a single layer on a baking sheet, then transfer to an airtight container. Use within 1 month. Cook just like fresh gnocchi, by dropping into a pot of boiling water.