Preheat the oven to 425°F. Line a rimmed baking sheet with aluminum foil or parchment paper. With a fork, poke holes in the outsides of the sweet potatoes. Bake for 40 to 60 minutes (timing will depend upon the size of your sweet potatoes), until very tender when pierced with a fork. Set aside to cool.
Reduce the oven temperature to 350°F. Coat a 9x13 casserole dish or similar 2 ½ quart casserole dish with nonstick spray.
Once the sweet potatoes are cool enough to handle, remove the peels. Scoop out the sweet potato flesh and place it in a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment. (You can also use an electric hand mixer.)
Add the butter, eggs, sugar, maple syrup, vanilla, cinnamon, salt, and milk.
Beat until smoothly combined.
Scoop into the prepared baking dish.
Next, prepare the topping. In a medium mixing bowl, melt the butter. Sprinkle the chopped pecans, coconut, brown sugar, flour, and salt over the top. Stir until evenly moistened and combined. Scatter the streusel topping over the sweet potatoes in an even layer.
Bake the sweet potato souffle until the topping is lightly browned and your kitchen smells amazing, about 40 minutes. Let rest 5 minutes, then enjoy warm.
TO STORE: Sweet potato soufflé may be kept in an airtight container for up to 4 days in the fridge.
TO REHEAT: Rewarm leftovers in a baking dish in the oven at 350°F or in the microwave.
TO FREEZE: Freeze souffle in an airtight, freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.
TO MAKE AHEAD: Up to 1 day in advance, prepare the filling and place it in the baking dish; prepare and refrigerate the topping separately. Cover and refrigerate until you're ready to add the topping and finish the recipe. You can also bake the dish 1 day in advance, then reheat in a 350°F oven.