The warmly spiced crunch of the pecan streusel contrasts nicely with the smooth and naturally sweet souffle. You can prepare the sweet potato mixture and the streusel up to 2 days in advance. Transfer the sweet potato mixture to a baking dish and let cool to room temperature, then cover tightly with plastic wrap and refrigerate. Store the streusel in an airtight container in the refrigerator. Before baking, let the sweet potato mixture stand at room temperature for 1 hour. Top with the streusel and bake as directed.