Preheat oven to 375°F. Line muffin tin with paper liners or grease very well with non-stick cooking spray. Set aside.
In a large bowl, gently whisk flour, brown sugar, baking powder, baking soda, cinnamon and salt together.
In a smaller bowl or large measuring cup, whisk buttermilk, oil, eggs and vanilla until completely combined. Make a well in the center of dry mixture and pour in liquid. Beat until fully combined. Batter will be slightly thick.
For the crumb topping, whisk together all dry ingredients in a separate bowl, then add the melted butter and stir until fully combined and crumbly.
In a small bowl, mix together the sugar and cinnamon.
Scoop 2 tbs of muffin batter into each muffin tin. Place 1 teaspoon of cinnamon sugar mixture on top of each. Then, cover with more batter, filling them ¾ to the top. Sprinkle crumb topping on top of each muffin distributing evenly.
Bake muffins for 15-17 minutes or until a toothpick inserted into the center comes out moist with a few crumbs. Allow them to cool in their pans for 10 minutes or until firm enough to handle before transferring to a wire rack to finish cooling completely.
While cooling, whisk together glaze ingredients. Drizzle on cooled muffins.