Preheat your oven to 425°F. Grease 12 standard muffin tins and line them with cupcake liners. Set them aside.
In a small bowl, prepare the streusel topping by combining the ingredients with a fork or pastry cutter until it resembles wet sand.
In a large mixing bowl, mix the melted butter and sugars until well combined. Add the eggs one at a time, followed by the Greek yogurt, milk, and vanilla extract.
Add the flour, baking powder, spices, and salt, mixing until it starts coming together. Gently fold in the apple pieces with a rubber spatula; the batter will be thick.
Scoop the batter into the prepared muffin tins, filling them about 3/4 of the way. You can use a large cookie scoop or a 1/4 cup measure for consistency.
Sprinkle about 1 tablespoon of the streusel mixture on top of each muffin, pressing it gently into the batter.
Bake at 425°F for 5 minutes, then reduce the oven temperature to 375°F and bake for an additional 15 minutes until the muffins are golden brown and spring back when touched.
While the muffins bake, you can make the icing by mixing all the icing ingredients until smooth and creamy.
Once the muffins have cooled completely, drizzle them with the icing, and they're ready to enjoy!