Preheat oven to 350 F.
Grease a 9x5 in loaf pan. If you have a 8x5 inch pan, you will need to bake the loaf an extra 5 minutes.
Cinnamon Swirl: Mix together 1/3 cup of brown sugar with 1 tbsp of cinnamon in a small bowl. Set aside.
Cinnamon Bread: Combine the all-purpose flour, brown sugar, baking powder, and salt in a medium sized bowl.
In a separate bowl, mix together the egg, milk, melted butter, vanilla, and sourdough discard.
Whisk the wet ingredients in with the dry ingredients using a sourdough hook. Stir until just moistened. Do not overmix.
Pour 1/2 of the batter into the greased loaf pan. Spread to the corners using a spoon or spatula.
Sprinkle 3/4 of the cinnamon sugar mixture on top.
Pour the remaining cinnamon bread batter on top. Use a spoon to gently spread towards the corners of the loaf pan. Sprinkle the remaining cinnamon sugar on top of the dough.
Draw a sharp knife through the batter in a 'S' swirl pattern to marble/swirl the batter.
Bake at 350 F on the middle rack until a toothpick comes out clean. 45 to 50 minutes.
Let the cinnamon bread cool on a wire rack for at least 20 minutes before adding glaze.
Optional glaze: Whisk together the powdered sugar, milk and vanilla until smooth. Drizzle over cooled loaf and enjoy.