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For topping: Add brown sugar, rolled oats, flour, walnuts, cinnamon and butter in a food processor (put the butter in last so it is on top).
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Pulse a few times until the mixture is coarse and crumbly (This can also be done by hand or with a pastry blender); set topping aside.
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Spray muffin pan with non-stick spray.
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In a bowl, whisk the buttermilk and eggs together.
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Add the melted butter and stir until well blended.
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In another bowl, mix together the flour, brown sugar, baking powder, baking soda, and cinnamon.
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Pour the buttermilk/butter mixture over the flour mixture, stirring just until moistened.
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Scoop 1/4 cup of batter into prepared muffin cups (will be almost full).
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Sprinkle evenly with the topping, covering the top of each muffin completely.
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Bake at 400 degrees for 15 to 18 minutes, or until a pick inserted in the center comes out mostly clean, with a few moist crumbs attached.
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Cool for 3 minutes and turn out on cooling rack.