To make the muffins:
Preheat oven to 400 degrees F (the higher temperature insures a crunchy top. Be sure to bake in the center of the oven.)
Prepare a muffin pan with 6 giant muffin cups or 12 normal sized papers.
In a large bowl whisk together the self-raising flour and nutmeg. In a separate bowl combine the melted butter, sugar, egg, milk, vanilla and ginger. Gently stir the wet into the dry ingredients only until combined. Gently fold in the blueberries. If using frozen berries, do this in as few stirs as possible or the batter will turn purple.
Divide the batter into the prepared muffin cups.
To make the topping:
In a bowl whisk together the flour, sugar, cinnamon and salt. Grate the butter over the top of the dry ingredients.
Quickly mix the butter into the flour mixture, which will break up the butter slightly, but it should still stay cold. If the butter warms up too much, stick the bowl in the freezer for about 5 minutes.
Divide the topping over the muffins.
Bake for about 30 minutes (about 20 minutes if using the smaller cups), or when golden on top and a tester comes out clean.
Allow to cool on a rack for 5 minutes.
Serve warm or room temperature.