When you need MASHED BANANAS to use in a recipe, let them thaw in a small bowl on the counter for at least 20-30 minutes. Then you can snip off the end (that attaches to the bunch) and just squeeze it into the bowl. It will be "pre-mashed" ~ if you let them sit for an hour or more, they'll actually be almost liquified.
If you don't want to wait, put the banana into your microwave for about 20 seconds on HIGH.