01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease with butter.
02 - In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - In a separate bowl, whisk melted butter and brown sugar until smooth. Add eggs one at a time, mixing well after each addition. Stir in sourdough discard, milk, and vanilla extract until incorporated.
04 - Add wet mixture to dry ingredients and stir with a spatula just until blended. Avoid overmixing.
05 - Gently fold diced apples into the batter.
06 - Divide batter evenly among prepared muffin cups, filling each about three-quarters full.
07 - Bake for 20 to 22 minutes, or until a toothpick inserted in the center comes out clean.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack.
09 - While muffins are still slightly warm, drizzle caramel sauce over each. Serve warm or allow to cool completely.