Preheat oven to 350°F (175°C).
Grease loaf pan thoroughly with butter.
Mix flour, sugar, baking powder, salt together.
Combine melted butter, egg, milk separately.
Pour wet ingredients into dry ingredients.
Stir until just combined, avoid overmixing.
Mix brown sugar, cinnamon, melted butter.
Pour half batter into prepared pan.
Sprinkle half cinnamon mixture over batter.
Add remaining batter, top with cinnamon.
Swirl gently with knife for patterns.
Bake 50-60 minutes until golden brown.
Cool in pan for 10 minutes.
Remove and cool completely on rack.