Prepare the muffin tin
Preheat the oven to 350°F. Line a 12-cup metal muffin tin with paper liners.
Mix the dry ingredients
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.
Mix the butter and sugar
In a large bowl, stir the melted butter and packed brown sugar until smooth and glossy.
Add the wet ingredients
Add the eggs, sour cream, milk, and vanilla. Whisk until the mixture is smooth and fully combined.
Combine the batter
Add the dry ingredients to the wet ingredients. Stir gently just until combined. A few small lumps are fine.
Make the topping
In a small bowl, mix the chopped pecans, brown sugar, and melted butter until the pecans are coated.
Fill the muffin cups
Divide the batter evenly among the lined muffin cups, filling each about three-quarters full.
Add the pecan topping
Spoon the pecan topping over each muffin. Press a pecan half into the center of each muffin, if using.
Bake
Bake for 18 to 22 minutes, or until the tops are set and a toothpick inserted in the center comes out clean.
Cool
Let the muffins cool in the pan for 10 minutes, then move them to a wire rack to cool completely.