Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and 1 teaspoon of cinnamon until well combined.
In another bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are okay.
In a small bowl, combine the brown sugar and 1 teaspoon of cinnamon for the topping.
Divide the muffin batter evenly among the muffin cups, filling each about two-thirds full. Sprinkle the cinnamon-brown sugar mixture generously on top of each muffin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.