Preheat your oven to 350°F (175°C) and grease a 9x5 inch loaf pan.

In a small bowl, combine 1/3 cup of brown sugar with 1 tablespoon of cinnamon, stir well and set aside.

In a medium bowl, whisk together 2 cups of all-purpose flour, 1/2 cup of brown sugar, 2 teaspoons of baking powder, and 1/2 teaspoon of salt until fluffy.

In a separate bowl, whisk together 1 large egg, 1 cup of milk, 1/4 cup of melted butter, 1 teaspoon of vanilla extract, and 1 cup of sourdough discard until smooth.

Pour the wet ingredients into the dry ingredients and gently mix until just moistened.

Spoon half of the batter into the prepared loaf pan and sprinkle 3/4 of the cinnamon sugar mixture evenly over it.

Pour the remaining batter over the cinnamon-sugar layer and sprinkle the last bit of cinnamon sugar on top.

Using a butter knife, swirl through the batter to create a marbled effect.

Bake for 45-50 minutes or until a toothpick comes out clean or with a few moist crumbs.

Let the loaf cool in the pan for at least 20 minutes, then transfer to a wire rack to cool completely.