Cinnamon Sugar Muffins:

1. Preheat oven to 450°F (232°C). Meanwhile, grease and line a 12-cup muffin pan with cupcake liners. Set aside.
2. In a large bowl, add the unsalted butter and brown sugar. Using a hand mixer or a stand mixer, beat on medium speed until creamy, about 2 minutes. Add in the eggs, one at a time and beat until combined.  Lastly add in the vanilla extract. Beat again until combined thoroughly. The mixture will now be light and fluffy.
3. In a separate bowl, whisk together the flour, ground cinnamon, baking soda, baking powder and salt until thoroughly combined.
4. Add the dry ingredients into the wet ingredients about 4-5 tablespoons at a time and beat on low speed until combined. Repeat until all dry ingredients are added. The batter will be slightly thick. Now, add the buttermilk into the batter. Beat on low speed until fully incorporated.
5. Pour or spoon the batter into the liners. Fill each liner upto ⅔ full to avoid it from spilling over the sides or sinking.

Cinnamon Streusel Topping:

1. In a separate bowl, whisk together the all-purpose flour, brown sugar, cinnamon, nutmeg and salt.
2. Add in the unsalted butter and mix until the mixture becomes crumbly in texture.
3. Spoon a few spoons of this mixture (around 2 teaspoons for each muffin) onto the muffin batter in the muffin pan. Spoon it evenly on top of the entire muffin to get a uniform streusel topping.

Assembly:

1. Bake muffins for 5 minutes at 450°F (232°C). This initial high temperature helps the muffins to rise and form the perfect dome shape. Reduce the temperature to 350°F (177°C) and bake the muffins for 14-15 minutes or until a toothpick inserted comes out clean. 
2. Transfer the muffin pan to a wire rack immediately when done and allow them to cool.
3. Store muffins in an airtight container along with some saltline crackers or paper towels at room temperature for upto a week.