Mix the brown sugar, butter, and cinnamon in a bowl until crumbs form. Chill in the freezer until ready to use.
Whisk together the egg, milk, vanilla, sour cream, melted butter, and sugar until combined.
Whisk the flour, baking powder, baking soda, and salt into the wet ingredients until almost combined.
Then fold the cinnamon crumb into the batter until just combined.
Scoop the batter into a 12 cup muffin pan and sprinkle the rest of the cinnamon crumb on top. Bake and enjoy!