Heat the oven to 400 F. Prick the sweet potatoes, set them on a sheet pan and roast 50 to 60 minutes until they collapse when pressed. Cool, peel and mash until smooth. You need 2 cups. Lower the oven to 350 F.
Pulse the oats, pecans, dates, salt and cinnamon in a food processor until it looks like coarse sand. Add the melted butter and pulse until it clumps. Press firmly into the bottom and up the sides of a 9-inch pie plate. Bake 10 minutes, then cool.
Melt the 4 tablespoons of butter in a small pan over medium heat and cook, swirling, 3 to 4 minutes until it foams, smells nutty and turns golden brown. Set aside.
Whisk the eggs, maple syrup, milk, vanilla, cinnamon, nutmeg, ginger and salt in a large bowl. Whisk in the mashed sweet potato and the browned butter until completely smooth. For a silkier pie, blend 20 seconds.
Pour the filling into the crust. Bake 45 to 50 minutes, until the edges are set and the center wobbles slightly. Tent the crust with foil if it browns too fast.
Cool on a rack at least 2 hours, then chill or serve at room temperature with the maple yogurt and toasted pecans.
