Preheat oven to 400°F. Line a 12-cup muffin pan with liners (or grease well).
Prep apples: Dice apples small. Toss with 1 tbsp flour and set aside.
Streusel (optional): Mix flour, brown sugar, cinnamon, salt. Stir in melted butter until clumpy. Refrigerate while you mix batter.
Dry bowl: Whisk flour, cinnamon, baking powder, baking soda, salt.
Wet bowl: Whisk eggs, brown sugar, granulated sugar, melted butter, milk, vanilla until smooth.
Pour wet into dry and fold just until combined (do not overmix). Fold in diced apples.
Cinnamon ripple (optional): Mix brown sugar and cinnamon. Fill cups: add a spoonful of batter, sprinkle a pinch of ripple, top with batter to 3/4 full. Add a little more ripple on top if desired.
Top with streusel (optional).
Bake 5 minutes at 400°F, then reduce to 350°F (keep door closed) and bake 12–16 minutes, until a toothpick comes out clean or with a few moist crumbs.
Cool in pan 5 minutes, then transfer to a rack.