Combine the brown sugar, flour, cinnamon and butter in a small bowl and mix well until until they have the consistency of wet sand. Set aside until ready to use.

Preheat the oven to 375F and line or grease 10 wells of a 12-cup muffin tin.

In a large mixing bowl, add the flour, cinnamon, baking powder, baking soda, and nutmeg.

Stir together to combine the dry ingredients.

In a medium mixing bowl, combine the sugar, egg, yogurt, vegetable oil, and buttermilk. Whisk until combined.

Pour the wet ingredients into the dry ones and mix until the ingredients are just combined.

Spoon the batter into the muffin tins until they are ¼ full-- this should use about half of the batter.

Divide half of the crumb layer between the muffins.

Top with the remaining batter, dividing between the muffin cups and top with the remaining crumb topping.

Bake the muffins for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool on a wire rack.

Once cool, store in an airtight container at room temperature for up to two days.