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

In a small bowl, mix together cinnamon and brown sugar for the swirl topping.

In a large bowl, whisk together all-purpose flour, brown sugar, baking powder, and salt.

In another bowl, whisk together egg, milk, melted butter, vanilla extract, and sourdough discard until smooth.

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

Pour half of the batter into the prepared loaf pan, sprinkle three-quarters of the cinnamon sugar mixture on top.

Spoon the remaining batter over the cinnamon sugar layer and sprinkle the rest on top, swirling gently.

Bake for 45-50 minutes, until a toothpick inserted in the center comes out clean.

Cool in the pan for 20 minutes, then transfer to a wire rack.

Optionally glaze with powdered sugar before serving.