To ripen a banana quickly for a snack, your best bet is to use the paper bag method, as heat-based hacks are better suited for baking.
For Immediate Snacking (Best Texture): Place your bananas in a brown paper bag with a ripe apple, avocado, or tomato. These fruits release ethylene gas, which triggers ripening; the bag traps this gas to speed up the process, usually ripening them within 24 to 48 hours. Using a plastic bag is not recommended as it traps moisture and can lead to mold.
For Baking (Breads/Muffins): If you need them "mushy" ripe right now for a recipe, you can use heat:
Oven: Bake unpeeled bananas at 300°F (150°C) for 15–30 minutes until the skins are completely black.
Microwave: Prick the unpeeled skin with a fork and microwave in 30-second intervals until soft.
Important: Heated bananas will be very soft, warm, and slightly less sweet than naturally ripened ones, making them less ideal for eating "out of hand".