Sweet Plantain Rum Cake with Maple-Walnut Streusel

For when the plantains go black and you don't want to fry them.

I eat up bananas quickly, most of the time, so they don’t always get brown. I like to put them in my oatmeal or into a grilled peanut butter and jelly. Plantains, though, go yellow in my tropical kitchen within a day or so. That’s how I long I have with the green ones, to turn into tostones, and so I often have yellow ones studded with brown to make as …