• Preheat your oven to 350°F (175°C).
• In a small saucepan, melt the butter over low heat. Once melted, remove from heat and set aside to cool slightly.
• In a large mixing bowl, whisk together the flour, brown sugar, white sugar, baking powder, and salt. Make a well in the center of the dry ingredients and set aside.
• In another bowl, combine the sourdough discard, eggs, vanilla extract, and melted butter. Stir to combine.
• Pour the wet ingredients into the well in the dry ingredients. Stir gently until just combined—don’t overmix. The batter will be thick. For deeper flavor, you can cover the batter and let it rest at room temperature for 30 minutes to 1 hour, or refrigerate it overnight. (If resting the batter, don’t assemble the bread until you’re ready to bake.)
• Liberally butter the inside of your loaf pan (9x5 inches), making sure to cover all surfaces. Then, dust the pan with flour to prevent sticking. If you'd prefer, you can also line the pan with parchment paper as an optional extra layer of protection.
• Prepare the Apple Cinnamon Mixture: Peel and dice the apples into 1/4-inch to 1/2-inch pieces. Toss them in a small bowl with the sugar and cinnamon until evenly coated.
• Make the Streusel Topping: In a small bowl, mix the brown sugar and cinnamon until well combined. No butter is needed for this streusel; the sugar and cinnamon will create a delightful, crumbly topping when baked.

Assemble the Bread:

• Spread half of the batter into the bottom of the prepared loaf pan.
• Top the batter with half of the apple cinnamon mixture, pressing the apples lightly into the dough.
• Sprinkle half of the streusel topping over the apples.
• Add the remaining batter, spreading it evenly to cover the apples and streusel.
• Top with the remaining apple cinnamon mixture and gently press the apples into the batter.
• Finally, sprinkle the remaining streusel evenly over the top of the bread.

Bake the Bread:

• Place the loaf pan in the preheated oven and bake for 60–70 minutes. The bread should have a golden-brown crust, and a toothpick inserted into the center should come out clean. If the top is browning too quickly, you can cover the bread loosely with foil during the last 20 minutes of baking.
• Cool the Bread: Allow the bread to cool in the pan for about 20 minutes before transferring it to a wire rack to cool completely. This helps the bread set and makes it easier to slice.
• Prepare the Vanilla Glaze: While the bread is cooling, whisk together the powdered sugar, vanilla extract, and 1–3 tablespoons of milk until smooth. Adjust the milk for your desired glaze consistency. If it’s too thick, add a bit more milk; if it’s too thin, add more powdered sugar.
• Glaze the Bread: Once the bread has cooled slightly, drizzle the vanilla glaze generously over the top. Let the glaze set before slicing the bread.
• Slice and serve! This bread is perfect with coffee, tea, or as a sweet treat for any occasion.