Preheat oven to 375°F. Make sure your oven is fully preheated before putting anything in — this ensures even baking from the very first minute.
Mix flour, sugar, baking powder, cinnamon, and salt. Always taste as you go — you can add more seasoning, but you can't take it away!
Whisk eggs, milk, and melted butter.
Fold wet into dry ingredients.
Fold in diced apples. Try to keep your pieces roughly the same size so everything cooks evenly.
Make streusel with brown sugar, butter, and flour.
Fill muffin cups, top with streusel. This is your chance to make it look as good as it tastes! A little garnish goes a long way for presentation.
Bake 18-20 minutes until golden. Every oven is a little different, so start checking a few minutes early. You're looking for a golden color and a wonderful aroma filling your kitchen.