Preheat oven to 375°F/190°C. Prepare muffin pans by lining with papers or spraying with vegetable cooking spray.
For the Crumble Topping:
Take a moment to mix the crumble topping ingredients - the vegan butter, gluten-free flour, brown sugar, and cinnamon. Stir well and set aside.
For the Banana Muffins:
Mash the bananas in a mixing bowl. Add the buttermilk, brown sugar and melted vegan butter. Stir until well combined and set aside.
Now combine the gluten-free flour, baking powder, baking soda, and salt. Stir until well combined. If using, add the chopped nuts to the flour.
Make the muffin batter by combining the wet and dry muffin ingredients. Stir until combined (some small lumps are ok).
Distribute the muffins in prepared muffin compartments.
Then equally sprinkle crumble topping over each muffin, using your hand to gently press it into the batter.
Place the pan in heated oven and bake for 20 - 22 minutes. Once they're done, remove them from the oven and set aside to cool for a few minutes before devouring.
For a dramatic flair, add a drizzle of vanilla frosting (1/2 cup powdered sugar combined with just enough soymilk to make it spreadable).