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In a large bowl, combine the brown sugar, egg, and oil. Mix well to combine.
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Add the vanilla and buttermilk and mix again.
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Slowly add the flour, baking soda, salt, and 1 scant teaspoon of the cinnamon. Mix until just incorporated.
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Stir in the rhubarb.
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Pour the batter into prepared muffin tins.
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In a separate bowl, mix together the melted butter, sugar, and remaining cinnamon.
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Sprinkle on top of the muffins.
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Bake at 350 degrees for 25-30 minutes.