Preheat oven to 375°F. Prepare muffin pans by lining with papers or spraying with vegetable cooking spray.
Next, mash the bananas in a medium 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 in a medium sized bowl. Stir until well combined. If using, add the chopped nuts to the flour.
Take a moment to mix together your topping ingredients - the vegan butter, gluten-free flour, and brown sugar. Stir well and set aside.
Make the muffin batter by combining the wet and dry muffin ingredients. Stir until combined (some small lumps are ok) and spoon into prepared muffin compartments. Then equally distribute 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 an even more dramatic flair, add a drizzle of vanilla frosting (1/2 cup powdered sugar combined with just enough soymilk to make it spreadable).