Preheat your oven to 175°C (350°F) and line a 12-cup muffin tin with paper liners.
Mix the dry ingredients: In a separate bowl, combine flour, both sugars, baking soda, baking powder, salt, and cinnamon.
Whisk the wet ingredients: In another bowl, combine discard (or starter), milk, oil, egg, and vanilla extract.
Combine wet and dry ingredients until just mixed — don’t overmix. Fold in diced apples.
Fill the muffin cups evenly with batter. Bake for 15 minutes.
Prepare the crumble: In a bowl, mix flour, brown sugar, and cinnamon. Add cold butter cubes and rub with your fingertips until the mixture resembles coarse crumbs.
Add the crumble topping, then bake for another 10 minutes, or until the crumble is golden and set.
Cool slightly before serving. Enjoy the soft apple-filled centers and the crisp, golden crumble top.