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1
Preheat the oven to 425u00b0 F.
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2
Line a 12-cavity muffin pan with muffin/cupcake liners.
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3
In large mixing bowl, use a hand mixer to cream together the butter, 1/2 cup granulated sugar, 1/4 cup light brown sugar (packed), and vanilla extract on medium-high speed for 1-2 minutes, or until smooth and combined. Add the eggs and beat for a minute longer, until the mixture is pale and fluffy. Add the sour cream and milk. Continue to beat until fully incorporated, scraping down the sides of the bowl with a rubber spatula.
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4
Add 1 3/4 cups flour, baking powder, baking soda, 1/2 tsp. cinnamon, and 1/2 tsp. salt to the bowl. Beat the mixture on medium-low speed, scraping down the sides of the bowl with a rubber spatula, until just combined. Do not over-mix.
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5
To make the streusel, place 1 1/8 cups flour, 2/3 cup light brown sugar (packed), 1 1/2 Tbsp. granulated sugar, 2 tsp. cinnamon and 1/4 tsp. salt in a large mixing bowl. Stir with a fork to combine. Drizzle the melted butter into the bowl, and mix with the fork until the mixture forms large crumbs.
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6
Fill each muffin liner approximately 1/3 full with muffin batter. Crumble a scant Tablespoon of streusel into each muffin cup. Divide the remaining batter into the muffin cups. Sprinkle the muffins with the remaining streusel.
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7
Transfer the muffin pan to the preheated oven. Bake the muffins at 425u00b0 F for 5 minutes.
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8
Reduce the oven temperature to 350u00b0 F. Continue baking for 14-16 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
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9
Remove the muffin pan from the oven. Allow the muffins to cool in the pan for 5 minutes before removing.
