Preheat oven to 350°F (175°C) and line muffin tin with paper liners.
Mix cinnamon and sugar in a small bowl; set aside.
In a bowl, cream butter and sugar until fluffy.
Add eggs one at a time, then stir in vanilla.
Alternate adding dry ingredients and sour cream, ending with flour.
Mix in milk slowly until smooth; don’t overmix.
Fill liners ¾ full and sprinkle with cinnamon sugar.
Bake 18–20 minutes until toothpick comes out clean.
Cool for 5 minutes in pan, then transfer to a wire rack.
Beat butter for frosting; add powdered sugar, cinnamon, vanilla, and cream.
Pipe or spread frosting on cooled cupcakes; sprinkle cinnamon sugar on top.