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Preheat the oven to 350°F. Line a muffin tin with 9 paper muffin liners.
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Make the batter: In large bowl, whisk together the white rice flour, potato starch, tapioca flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
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In another bowl, whisk together the butter, maple crystals, egg, bananas, vanilla, and buttermilk.
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Whisk the wet ingredients into the dry ingredients. Fold together, making sure to scrape the bottom of the bowl to mix well. Divide the batter evenly in the 9 cups of the prepared muffin tin.
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Make the crumble: In a small bowl, mix together the butter, maple crystals, and rolled oats. Sprinkle the crumble evenly over the 9 batter-filled cups.
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Bake until a toothpick inserted into the center of a muffin comes out clean, about 30 minutes. Let cool in the pan for 15 minutes before transferring the individual muffins onto a wire rack to cool completely.