Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.

In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.

In a large mixing bowl, mix melted butter with packed brown sugar until smooth. Add eggs, sour cream, milk, and vanilla; stir until fully combined.

Gently fold the dry ingredients into the wet ingredients until just combined.

In a small bowl, mix together the chopped pecans, additional brown sugar, and melted butter.

Spoon the muffin batter into prepared muffin cups, topping with pecan mixture and pressing a pecan half into each muffin.

Bake for 18-22 minutes or until a toothpick comes out clean.

Let the muffins cool in the pan for 10 minutes before transferring them to a wire rack.