Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease thoroughly.
Peel and core your apples, then dice them into small 1/4-inch pieces.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, and salt.
In a separate bowl, whisk together the sourdough discard, eggs, oil, milk, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined – about 12-15 strokes. Fold in the diced apples with just 3-4 additional strokes.
In a small bowl, mix the brown sugar, flour, and cinnamon for the topping. Cut in the cold butter using a fork or your fingertips until the mixture resembles coarse crumbs.
Divide the batter evenly among the muffin cups, filling each about 3/4 full.
Sprinkle the streusel topping generously over each muffin. Bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs. Rotate the pan halfway through baking.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.