Heat oven to 350°F. Grease a 9x5-inch loaf pan and line with a parchment sling.
Whisk flour, granulated sugar, baking powder, baking soda, salt, and 1 tsp cinnamon in a large bowl.
In another bowl, beat milk, oil, eggs, and vanilla until smooth.
Fold wet mixture into dry just until combined; do not overmix.
Stir together brown sugar, 1.5 tbsp cinnamon, and melted butter to make the swirl.
Spread half the batter in the pan. Sprinkle on half the cinnamon swirl. Add remaining batter and top with remaining swirl.
Drag a knife through the layers in a gentle S pattern to marble.
Bake 50–55 minutes, until a tester comes out clean or with a few moist crumbs.
Cool in pan 15 minutes, then lift out to a rack. Cool completely before slicing.