Line your muffin tins with muffin cups, or simply grease them if you prefer.
Sift the flour, baking powder, salt, and sugar together in a mixing bowl. Make a well in the center, and set aside.
In another bowl, combine the milk, egg, and melted butter, stirring to mix up the egg.
Pour the wet ingredients into the well of the dry ingredients and stir until just barely combined, careful not to over-mix.
Add half of the topping mixture and fold it in; this will finish mixing the muffins too.
Fill your muffin cups with about a quarter of a cup of batter each, and top with the remaining topping.
Bake for 13-15 minutes, or until a toothpick inserted comes out mostly clean.
Remove from the oven and let cool for 5 or so minutes while you prepare the icing.