In a large bowl, cream together the shortening, brown sugar and vanilla. Add the eggs one at a time and combine. Add the flour, salt, baking powder, rolled oats, cinnamon, nutmeg and combine. Add the chopped apples and fold in.
In a mixing bowl, combine the butter, brown sugar, flour and ground cinnamon with your fingers to create the streusel topping.
Scoop the batter into paper-lined muffin tins. Sprinkle with streusel topping.
Bake in a preheated 350 degree F oven for about 25 minutes or until a toothpick comes out clean.
In a small bowl, whisk together the icing ingredients until smooth. Cover with plastic wrap and set aside.
Let the muffins cool slightly and drizzle with maple icing.