1 cup (2 sticks) unsalted butter, at room temperature
1/4 cup honey
3/4 cup dark brown sugar
1/2 cup all-purpose flour
1 1/2 cups chopped pecans
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.
Preheat the oven to 400 degrees F. Prick the sweet potatoes all over with a fork, put them in a roasting pan and roast for 30 minutes. Toss the bananas into the pan and continue roasting for 10 to 15 minutes, until both the bananas and potatoes are very soft. Remove the pan from the oven but don't turn the oven off.
When the potatoes are cool enough to handle, scoop out the flesh into a large mixing bowl. Peel the bananas and add them to the bowl along with 1 stick of the butter, and the honey. Season with salt and beat vigorously with a wooden spoon until everything's well combined and the mixture is fluffy. Spoon into an oven-proof serving bowl and smooth the top.
In a separate mixing bowl, use your fingers to rub together the remaining stick of butter, the brown sugar, flour, and pecans until the mixture is the consistency of coarse crumbs. Sprinkle the crumb mixture over the sweet potatoes and return to the oven. Cook for about 20 minutes, until the crumbs are golden. Serve hot.