Adjust oven rack to middle position and heat oven to 375 degrees.
Spray standard muffin tin with nonstick cooking spray.
Whisk together flour, baking soda, salt, ginger, cinnamon, and allspice in medium bowl until combined.
Whisk together eggs, brown sugar, and buttermilk in large bowl until combined. Add melted butter and whisk vigorously until thick and homogeneous.
Whisk half of dry ingredients into wet ingredients until two begin to come together and then add remaining dry ingredients and stir with rubber spatula or wooden spoon until just combined (do not overmix).
For Topping: Process crystallized ginger with sugar in food processor until finely ground.
Use ice cream scoop or large spoon to drop batter into greased muffin tin.
Sprinkle portion of ginger sugar over each muffin. Bake until golden and toothpick inserted into center of muffin comes out clean, 18 to 22 minutes, rotating pan from front to back halfway through baking time.
Cool muffins in tin for 5 minutes and then transfer to wire rack and cool for 10 more minutes. Serve warm.