Open the sweet potato cans. Reserve 1 cup of liquid and then drain the rest of the syrup from the sweet potatoes. Put sweet potatoes into a 2-quart glass casserole dish. Don't cut up the potatoes unless they're really big. (I like to use an 8.5x11 dish, but a 9x9 dish will work fine if that's what you have on hand.)
Dot the top of the sweet potatoes with pats of butter.
Sprinkle 3/4 cup of brown sugar on top of the sweet potatoes and butter. Pour enough of the reserved sweet potato syrup to fill the bottom 1/4-inch of the casserole dish. Then add the maple syrup and stir gently to coat.
Top with the remaining 1/4 pound of brown sugar.
Bake for 1 hour and 30 minutes. The sweet potatoes will release juices as they cook. Be sure to extract excess liquid from the dish with a baster so that the casserole doesn't boil over. (see note)
Remove from the oven and gently stir the sweet potatoes. Let rest for 5 minutes. Then sprinkle a little extra brown sugar on top, if desired, and serve warm.
Notes
You can use real maple syrup if you like. That can be a bit pricey, so just know that maple-flavored syrup works just fine for this recipe, too.
I recommend putting a baking sheet under the casserole dish in case anything does happen to boil over.
To Make Ahead: Bake the sweet potatoes, cool them to room temperature, and then cover and refrigerate them. The next day, let the dish warm up on the counter for a bit, and then reheat the sweet potatoes in the oven at 325 degrees F until warmed through.