Preheat your oven to 350°F (175°C) and prepare a 9x5 inch loaf pan by greasing it or lining it with parchment paper.

In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon.

In a medium bowl, whisk together the milk, vegetable oil (or melted butter), eggs, and vanilla extract until smooth.

Pour the wet mixture into the dry ingredients and gently stir until just combined.

Pour half of the batter into the prepared loaf pan, smoothing the top with a spatula.

In a small bowl, combine the brown sugar, ground cinnamon, and softened butter until crumbly.

Sprinkle the cinnamon swirl mixture on top of the first layer of batter.

Spoon the remaining batter on top of the cinnamon swirl layer and smooth it out.

Using a knife, gently swirl through the top layer of batter to create a marbled effect.

Bake in the preheated oven for 50 to 60 minutes, checking for doneness with a toothpick.

Allow the bread to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.