Preheat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or spray it.
In a large bowl, whisk melted butter and brown sugar.
Add eggs, vanilla, and rum extract (or light rum). Swirl till smooth.
Put in milk. Swirl to combine.
In another bowl, mix flour, baking powder, baking soda, salt, and cinnamon (if using).
Add dry ingredients to wet ingredients. Whisk gently. Batter should be a tiny bit lumpy.
Scoop batter into muffin cups, stuffing each about 2/3 full.
Bake for 18–22 minutes. Tops should be golden and soft.
Let muffins cool for 5 minutes in the pan. Move to a wire rack. Serve warm.