Put cubed potatoes into steamer basket and place steamer basket into a large pot of simmering water that is no closer than 2 inches from the bottom of basket. Allow to steam for 20 minutes or until the potatoes are fork tender. Mash with potato masher and set aside.
Preheat the oven to 350 degrees F.
Place sweet potatoes in the bowl of a stand mixer and beat with the paddle attachment. Add yogurt, brown sugar, cinnamon, nutmeg, yolks, and salt, to taste, and beat until well combined. Pour this batter into the pie shell and place onto a sheet pan. Sprinkle pecans on top and drizzle with maple syrup.
Bake for 50 to 55 minutes or until the custard reaches 165 to 180 degrees. Remove from oven and cool. Keep refrigerated after cooling.
Recipe courtesy of Alton Brown