Preheat oven to 350 degrees Fahrenheit.

Put liners in your muffin tins and spray with nonstick spray.

In a large bowl, mix together the cake mix, oil, sour cream and eggs until most lumps are gone.

Fill the cupcake tins with 1 heaping tablespoon of batter.

In another bowl, combine the brown sugar and the cinnamon.

Sprinkle a generous amount of the sugar mixture over the batter in each tin.

Drop 1 tablespoon of batter on top of the sugar mixture in the tins.

Use a butter knife to swirl the batter and sugar mixture together.

Bake for 27-30 minutes or until a toothpick comes out clean.

In the last minutes of baking time, in a bowl, mix together the powdered sugar, milk and vanilla.

Remove the liners from the muffins if preferred and drizzle the icing over the tops.

Garnish with extra cinnamon if desired.