Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the dry ingredients: flour, sugar, baking powder, cinnamon, and salt. Stir well.
In a separate bowl, whisk together the wet ingredients: melted butter, milk, eggs, and vanilla extract until smooth.
In a small bowl, mix the streusel topping ingredients: flour, brown sugar, and cinnamon. Add cold butter and mix until crumbly.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Fold in the diced apples.
Spoon the batter into a prepared muffin tin, filling each cup about two-thirds full. Sprinkle streusel topping over each muffin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick comes out clean. Let cool for 5 minutes before transferring to a wire rack.