Special equipment: a 6-cup large muffin pan and 6 large paper liners (or a 12-cup regular muffin pan with 12 regular paper liners)
To make the muffins: Position an oven rack in the center of the oven and preheat to 400 degrees F. (The higher temperature insures a crunchy top. Be sure to bake in the center of the oven.) Prepare a 6-cup large muffin pan with 6 large paper liners, or a 12-cup regular muffin pan with 12 regular paper liners.
In a large bowl, whisk together the self-rising flour and nutmeg. In a separate bowl, combine the melted butter, brown 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 all-purpose flour, granulated 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 for the large cups (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.