Prepare the crumb topping: In a medium bowl, combine all-purpose flour, granulated sugar, ground cinnamon, and a pinch of salt. Pour in the melted unsalted butter and mix until the mixture resembles coarse crumbs.

Make the cinnamon sugar swirl: In a small bowl, mix together granulated sugar and ground cinnamon until well combined.

Prepare the muffin batter: Preheat your oven to 400°F (204°C) and line a muffin tin. In a large mixing bowl, whisk together all-purpose flour, baking powder, ground cinnamon, and salt.

Cream the butter and sugars: Beat the unsalted butter along with granulated sugar and light brown sugar for about 3 minutes until fluffy. Add the eggs one at a time, mixing well, then stir in vanilla extract, buttermilk, and sour cream.

Combine wet and dry ingredients: Gently fold the dry mixtures into the wet ingredients, mixing just until combined.

Assemble the muffins: Fill each muffin cup halfway, layer with cinnamon sugar, then top with more batter and crumb topping.

Bake the muffins: Bake for 20-22 minutes until golden and a toothpick comes out clean. Cool in the pan for 10 minutes.

Prepare the glaze: Whisk together confectioners' sugar, buttermilk, vanilla extract, and a pinch of salt until smooth.