Place the cinnamon, brown sugar, and flour in a medium bowl.
Use a whisk or small spatula to mix the ingredients together.
Add the melted butter and stir until moistened. Set aside while you prepare the muffin batter.
To cream the butter properly you need to start with room temperature butter. You know it’s the right consistency when you can easily make an indent in the butter with a gentle press of your finger. Add the sugar and with a hand mixer on medium speed beat until the butter appears light and fluffy, usually less than 2 minutes.
Add the lightly beaten egg and mix well.
Add the sour cream.
Add the vanilla,
Mix with a hand mixer on low for about 30 seconds.
In a separate medium bowl, whisk together the flour, baking powder, and salt.
Use a wooden spoon or spatula and stir in dry ingredients into the wet ingredient.
Stir in the milk. The batter will be thick.
Add a small scoop of batter to the bottom of the paper cupcake liner.
Top with about a teaspoon of streusel mix.
Fill the muffin liners ¾ full. Evenly divide the remaining streusel over the 6 muffins.
Bake for 15-18 minutes. The tops of the muffins should be a light golden brown. If a toothpick inserted in the center of one of the muffins comes out clean., the muffins are done.
Cool the muffins in the pan for about 10 minutes, and then carefully transfer them to a cooling rack.