Preheat your oven to 350°F (177°C). Lightly grease or line a 9x5-inch loaf pan with parchment paper or use a buttered loaf pan. Set aside.
In a small bowl, combine the brown sugar and cinnamon. Set aside.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon.
In another large bowl, whisk the eggs, granulated sugar, oil, milk, sourdough discard, and vanilla extract. Mix until fully combined. Pour the wet mixture into the dry ingredients. Stir gently using a wooden spoon or spatula until just combined. Be careful not to overmix. The batter should look slightly lumpy.
Pour ⅓ of the batter into your prepared loaf pan. Sprinkle ½ of the cinnamon sugar mixture evenly on top. Repeat with half of the leftover batter and the remainder cinnamon sugar. Finish with remaining batter. Swirl a few times with a knife to create a marbled effect. Don’t over-swirl!
Bake for 50–60 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean. Tent loosely with aluminum foil halfway through to prevent over-browning.
Let the bread cool in the pan on a wire rack for 1 hour before removing. If using, whisk together the optional glaze ingredients and drizzle over the top of the loaf once fully cooled.