Preheat oven to 425?F. Grease 10 standard size muffin tins and line with cupcake liners. Set aside.

In a large mixing bowl combine the melted butter and granulated sugar, beating until combined. Add the eggs in one at a time. Add the sour cream, buttermilk and vanilla extract.

Add the flour, baking powder, and salt. Mix until it begins to come together (not fully incorporated). Set aside.

In a small bowl combine all of the ingredients for the brown sugar swirl. Stir to combine and set aside.

In a separate bowl, combine all of the dry ingredients for the crumb topping. Add the butter and mix together until the mixture clumps together. Set aside.

Use a large spoon to drop coffee cake muffin batter into prepared muffin tins, filling them up about 1/2 of the way. Drop a large spoonful of the brown sugar mixture over the bottom layer of the muffins. Top the brown sugar middle with the same amount of muffin batter to fill the muffins up about 3/4 of a way.

Top each muffin with a large spoonful of crumb topping, gently pressing into the top of the muffin so that it sticks.

Bake for 5 minutes at 425?F, then turn oven down to 375?F and bake for an additional 15 minutes, until muffins are golden brown and spring back to the touch. Remove from oven and allow to cool for at least 15 minutes.

While the muffins cool, make the glaze. Combine all of the glaze ingredients in a bowl and whisk together until smooth. Drizzle the glaze over the crumb topping.