Preheat oven 375°. Grease a 12-cup muffin tin with vegetable oil spray or line with muffin liners.
Make the streusel topping: whisk together the flour, sugar, cinnamon, salt, and oats. Stir in melted butter until incorporated. Using clean fingers, mix together until it resembles coarse crumbs. Set aside.
For the muffins: mix the flour, sugar, baking powder, baking soda, salt and spices together in a large bowl. Whisk the eggs, milk, yogurt and vanilla extract in a medium bowl. Gently fold the yogurt mixture into the flour mixture with a rubber spatula until just combined. Fold in diced apple and melted butter.
Using a large ice cream scoop, divide the batter evenly among the muffin cups. Crumble streusel topping over each muffin top.
Bake 20-25 minutes or until golden brown and a toothpick inserted into the center of a muffin comes out mostly clean. Let muffins cool in pan for 5 minutes then remove from pan and place onto wire rack. Serve warm and save leftover muffins in air tight container.