For yams:

Butter 13x9x2-inch glass baking dish. Peel yams; cut 1 inch off ends. Coarsely grate enough of yam ends to measure 1/2 cup; set aside for streusel. Discard remaining ends. Cut remaining whole yams into 1/3-inch-thick rounds. Arrange yam rounds in 4 overlapping lengthwise rows in prepared baking dish.

Bring 1/2 cup water, maple syrup, butter, and cider vinegar to boil in small saucepan. Stir in 1 teaspoon coarse salt and 3/4 teaspoon pepper. Pour maple syrup mixture over yams. Cover baking dish tightly with foil.

For streusel:

Mix first 4 ingredients in small bowl. Add melted butter and rub in with fingertips until moist clumps form. Mix in reserved 1/2 cup grated yams. DO AHEAD: Yams and streusel can be made 6 hours ahead. Cover separately and refrigerate. Let yams stand at room temperature 1 hour before baking.

Preheat oven to 375°F. Bake yams covered until almost tender, about 40 minutes. Increase oven temperature to 400°F. Sprinkle streusel over and bake uncovered until yams are very tender and streusel is golden and slightly crisp, about 35 minutes longer. Serve warm.