Preheat the oven to 350°F (175°C).
Line a 12-cup muffin tin with cupcake liners.
In a large bowl, whisk together sourdough discard, avocado oil, eggs, vanilla extract, and maple syrup until well combined.
In a medium bowl, mix the chopped apples with flour, baking powder, brown sugar, cinnamon, and salt until apples are well coated.
Add the apple and flour mixture to the wet ingredients. Stir using a spatula until just combined. Avoid overmixing to keep the muffins tender.
Spoon the batter into the prepared muffin cups, filling each about 3/4 full.
In a small bowl, combine the melted butter, brown sugar, and cinnamon. Drizzle this mixture over the top of each filled muffin cup.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.