Preheat your oven to 350°F (175°C) and grease a 9×5-inch loaf pan.
In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
In another bowl, mix together the milk, vegetable oil, egg, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, stirring just until combined. Be careful not to overmix.
In a separate small bowl, mix together the softened butter, brown sugar, and cinnamon until you have a smooth, spreadable cinnamon mixture.
Pour half of the bread batter into the prepared loaf pan. Drop spoonfuls of the cinnamon mixture on top of the batter, then swirl gently with a knife to create a marbled effect. Repeat with the remaining batter and cinnamon mixture.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.
While the bread is cooling, whisk together the powdered sugar, milk, and vanilla extract to make the glaze.
Drizzle the glaze over the cooled bread before slicing and serving.
Variations & Tips
For a nutty twist, try adding a handful of chopped pecans or walnuts to the cinnamon mixture. If you have picky eaters who prefer less sweetness, you can reduce the amount of brown sugar in the cinnamon mix. For a fall-inspired version, sprinkle a little nutmeg or add a bit of pumpkin puree to the batter for a cozy seasonal flavor.
