Preheat oven to 425°F. Grease 12 standard size muffin tins and line with cupcake liners. Set aside.

In a small mixing bowl, combine all the ingredients for the streusel, using a fork or a pastry cutter to stir together until the mixture resembles wet sand.

In a large mixing bowl combine the melted butter and sugars, beating until combined. Add the eggs in one at a time. Add the Greek yogurt, milk and vanilla extract.

Add the flour, baking powder, spices and salt. Mix until it begins to come together (not fully incorporated). Add the apple pieces and use a rubber spatula to fold everything together. Batter will be thick.

Scoop batter into prepared muffin tins, filling about ¾ of the way. You can use a large cookie scoop or a ¼ cup measure to keep things consistent.

Sprinkle the tops of each muffin with about 1 Tablespoon of the streusel mixture, pressing the streusel gently into the batter so it sticks.

Bake for 5 minutes at 425°F, then turn oven down to 375°F and bake for an additional 15 minutes, until muffins are golden brown and spring back to the touch.

If desired, make the frosting while the muffins bake. Combine all the ingredients for the icing in a bowl and mix together until smooth and creamy.

Remove the muffins from the oven and allow to cool completely before drizzling with icing and eating!