Preheat your oven to 350°F (175°C) and grease a 9×5 inch loaf pan.

Mix 1/3 cup of brown sugar with 1 tablespoon of ground cinnamon in a small bowl and set aside.

In a medium bowl, whisk together 1 3/4 cups of all-purpose flour, 1 cup of brown sugar, 2 teaspoons of baking powder, and 1/2 teaspoon of salt.

In another bowl, whisk together 1 large egg, 1/2 cup of milk, 1/4 cup of melted butter, 1 teaspoon of vanilla extract, and 1 cup of sourdough discard.

Fold the wet ingredients into the dry ingredients until just combined.

Pour half of the batter into the prepared pan and sprinkle 3/4 of the cinnamon-sugar mixture on top.

Add the remaining batter and then sprinkle the rest of the cinnamon-sugar mixture on top.

Swirl the mixture gently with a knife to create a marbled effect.

Bake for 45 to 50 minutes, or until a toothpick comes out clean.

Let the bread cool for 20 minutes before transferring to a wire rack to cool completely.