Mix together the flour, brown sugar, and cinnamon. Add 5 individual tablespoons of cold butter. Cut the butter into the mixture using a pastry cutter or fork until it resembles wet sand with some small chunks. Place this in the refrigerator while you make the batter.
Preheat oven to 350°F and line a cupcake pan with 12 liners (or spray with oil if you don't have liners).
In a large bowl, cream together the butter and sugar until light and fluffy (~3 minutes). Add the eggs, one at a time, followed by the vanilla and sour cream. Mix well.
In a separate bowl, whisk together the flour, baking soda, baking powder, and salt.
Gently stir half of the dry ingredients into the wet ingredients, then mix in the buttermilk. Add the remaining dry ingredients and mix until only a few flour streaks are visible. The batter will be pretty thick.
Pour 1 teaspoon of the cinnamon streusel into the bottom of each cupcake liner. Then, add a tablespoon of muffin batter over the top of it, followed by 2 teaspoons of streusel. Repeat until the muffin liner is about ¾ full. Try to spread the batter out so it's smooth and then finish by covering the tops with streusel. (Optional: sprinkle a little turbinado sugar on the tops too!)
Bake for 13-18 minutes or until the tops are lightly tan and a toothpick inserted in the center of one comes out clean.
Allow the muffins to cool in the pan for 5-10 minutes, and then carefully transfer them to a cooling rack.
*To make your own buttermilk, mix ¼ cup of milk with ¾ teaspoon of vinegar or lemon juice. Let it sit for a few minutes to curdle.