01 - Preheat oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and cinnamon until well blended.
03 - In a separate bowl, whisk together eggs, buttermilk, vegetable oil, vanilla extract, and melted butter until smooth and completely incorporated.
04 - Pour wet mixture into the dry ingredients and gently mix just until combined. Avoid overmixing to ensure a tender crumb.
05 - Using a scoop or spoon, divide the batter evenly among the prepared muffin cups, filling each approximately three-quarters full.
06 - In a small bowl, use a fork to blend together brown sugar, flour, cinnamon, softened butter, and chopped pecans (if desired) until the mixture resembles coarse crumbs.
07 - Sprinkle the cinnamon crunch mixture generously over each portion of muffin batter.
08 - Place muffin tin in the oven and bake for 16 to 18 minutes, until a toothpick inserted into the center of a muffin comes out clean.
09 - Remove the pan from the oven and allow muffins to cool in the tin for five minutes, then transfer to a wire rack to cool completely.