Preheat the oven to 400 degrees F. Prick the sweet potatoes all over with a fork and put them on a roasting pan. On another roasting pan prick bananas and lay out in a single layer. Roast sweet potatoes for 45 minutes until very soft and roast bananas for 25 minutes. Remove both pans from the oven and allow to cool slightly.
In a small sauce pot heat cream with bay leaf until reduced by half, then keep warm until ready to puree potatoes and bananas.
When the potatoes are cool enough to handle, scoop out the flesh into the bowl of a food processor. Add peeled bananas. Season with salt, add butter, cream, cinnamon, nutmeg, vanilla and puree until super smooth. Pour into an oven-proof gratin dish.
Make the topping by combining flour, cold butter and brown sugar in the food processor and pulse until you have the texture of coarse bread crumbs. Sprinkle on top of the puree and place under the broiler until golden brown and crispy, about 4-5 minutes.
You can roast the potatoes and bananas ahead of time, then put it all together just before serving. Then heat it up in the oven.