To make the muffins: Preheat oven to 375 degrees F (190 degrees C). Lightly grease 12 muffin cups, or line with muffin papers, set aside.
In a large bowl, combine 1½ cups flour, baking soda, baking powder, nutmeg, cinnamon, and salt.
In another bowl, beat together bananas, sugar, egg and melted butter (or oil). Stir the banana mixture into the flour mixture just until moistened then spoon batter into prepared muffin cups.
In a small bowl, mix together brown sugar, flour and cinnamon and butter until the mixture resembles coarse cornmeal. Next, Sprinkle a generous layer of topping over muffins.
Bake in preheated oven for 18 to 20 minutes, until a toothpick inserted into center of a muffin comes out clean. Cool for 15-20 mins until fully cooled before piping in the frosting.
To make the cream cheese frosting:In a large bowl, beat together the butter and cream cheese with an electric mixer until pale and creamy.Lower the speed and add the powdered sugar a cup at a time until smooth and fluffy Beat in the fresh lemon juice and lemon zest.
Prepare your pping bag with a large star or round tip such as the 230. Fill the piping bag with the frosting. Insert the tip in top of cupcake and squeeze until frosting begins to almost burst out of the hole. Enjoy!