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Preheat oven to 375 degrees F.
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Line a muffin tin with paper cups and set aside.
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In a medium bowl combine flour, baking powder, baking soda, cream of tartar, cinnamon and salt. Stir and set aside.
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Add butter and both sugars to a large bowl and beat until light and creamy. Stop to scrape the sides and bottom of the bowl with a spatula.
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Add egg, sour cream, vanilla and milk. Beat until fully combined and smooth. Stop to scrape the sides and bottom of the bowl with a spatula.
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Add dry ingredients to the wet ingredients, and using a spatula fold them in. You don't have to use the mixer for this step, to avoid over-mixing the batter.
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Once the flour mixture has been fully incorporated, scoop the batter into the prepared muffin cups.
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Brush the muffins with melted butter and sprinkle cinnamon sugar on top.
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Bake the muffins for 15-25 minutes. It varies based on the oven, but start checking on them after 15 minutes. The muffins are done when a toothpick inserted in the center comes out clean or with just a few crumbs, but NOT raw batter.
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Remove muffins from oven when ready and place pan on a cooling rack.
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If desired, brush muffins again with melted butter and sprinkle or roll in extra cinnamon sugar. This should be done while the muffins are still warm.
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Cool muffins in the pan for 10 minutes before removing them.
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Serve warm or at room temperature.