Put the sweet potatoes in a steamer basket and put the steamer basket in a large pot of simmering water that is 1 inch from the bottom of the basket. Cover and steam over medium-high heat until the sweet potatoes are fork-tender, about 20 minutes.
Remove the steamer basket and dump the sweet potatoes into the bowl of a stand mixer. Mash with a potato masher.
Heat oven to 350ºF.
Beat the sweet potatoes in a stand mixer fitted with the paddle attachment until smooth. Add the yogurt, brown sugar, cinnamon, nutmeg, egg yolks, and salt, and beat until well combined.
Set the pie shell (still in its pie tin) on a half-sheet pan, fill with the sweet potato mixture, and smooth the top. Sprinkle the pecans on top and drizzle with the maple syrup.
Bake until the pie reaches an internal temperature of 165 to 175ºF — the middle should still wiggle slightly — 50 to 55 minutes. Remove from the oven and cool on a wire rack for 1 hour. Slice and serve. Stash leftovers in the refrigerator.