Whisk together the flour, salt, baking powder and spices into a bowl and set aside.
In a large bowl, mix together the wet ingredients (sugar, egg, oil, butter, greek yogurt, whole milk and vanilla) with a whisk until combined.
Add the wet ingredients into the dry ingredients and mix with a rubber spatula or wooden spoon until just combined. Fold in the diced apple until incorporated, but do not overmix.
Add the batter to the fridge for 30 minutes (if time allows).
Preheat the oven to 425℉ and line a standard muffin tin with 12 liners. You can also omit the liners and simply grease your tin.
Fill each well ¾ full with batter and evenly distribute the streusel topping over each muffin.
Bake for 5 minutes, then lower the oven temperature to 350℉ and bake for another 13-16 minutes until a toothpick inserted into the center of the muffin comes out clean.
Cool for 5 minutes in the pan, then remove the muffins to a wire rack to finish cooling.