Preheat oven to 400°F and line a 12-muffin tin. I use liners to skip scrubbing.
In one bowl, whisk flour, sugar, baking powder, salt, and cinnamon.
Add egg, milk, oil, and vanilla. Stir until just mixed. Batter will be lumpy.
Fold in chopped apple gently.
Make streusel: mix brown sugar, cold butter, and oats with a fork until crumbly.
Spoon batter into cups. Sprinkle streusel on top.
Bake 18–22 minutes until tops are golden and a toothpick comes out mostly clean.
Cool 5 minutes in pan, then move to a rack.