• Preheat the oven to 425 degrees F. Line a 12-cup muffin tin with paper liners.

Make the cinnamon sugar and streusel

• Mix the brown sugar and cinnamon for the cinnamon swirl together in a small dish. Set aside
• In a small bowl, mix the flour, brown sugar and cinnamon for the streusel topping. Add the melted butter and stir it in using a fork until the streusel is wet and forms clumps.
• Sprinkle the streusel topping evenly over each muffin.

Make the muffin batter

• Add the flour, baking soda, baking powder, salt and cinnamon to a large mixing bowl. Stir to combine the dry ingredients.
• In a separate mixing bowl, combine the melted butter, granulated sugar and brown sugar. Stir well until the sugars and butter are well combined.
• Add the eggs to the wet ingredients and whisk them in.
• Add the buttermilk, sour cream and vanilla to the wet ingredients. Stir until the mixture is smooth with no lumps.
• Add the wet ingredients to the dry ingredients. Fold them together just until the batter is nice and smooth.

Assemble the muffins

• Using a small cookie scoop, scoop about 1.5 tablespoons of batter into each muffin liner.
• Sprinkle the cinnamon sugar mixture you made earlier evenly onto each muffin, forming a cinnamon sugar layer over the muffin batter.
• Spoon another 1.5 tablespoons of batter into each muffin tin over the cinnamon sugar layer.

Bake the muffins

• Bake the muffins at 425 degrees F for 5 minutes. After 5 minutes, turn the oven down to 350 degrees F. Continue to bake the muffins for 10-15 minutes, until a toothpick inserted into the center comes out clean.
• While the muffins bake, make the vanilla icing. Mix together the powdered sugar, vanilla and milk in a small bowl until no lumps remain.
• When the muffins are done, remove them from the oven and allow them to cool for at least 10 minutes before removing from the tin. Spoon icing over the muffins and enjoy!