Preheat the oven to 325°F [165°C]. Line a 12-well standard or 6-well jumbo muffin pan with paper liners or coat thoroughly with nonstick cooking spray.
In a large bowl, whisk together the melted butter and the eggs. Add the buttermilk, molasses, brown sugar, and vanilla and whisk until well combined. In another large bowl, whisk together the flour, baking powder, baking soda, salt, ground ginger, cinnamon, allspice, cloves, and black pepper. Stir in the chopped crystallized ginger.
Add the flour mixture to the butter mixture and use a rubber spatula to carefully fold together until just combined. Be careful not to overmix, or your muffins will be tough; the batter should still have a couple of streaks of flour.
Divide the batter equally among the prepared muffin wells. Because these muffins have a lot of moisture, they are prone to collapsing a bit in the center. To prevent this, do not fill the muffin wells more than two-thirds full (if you have a little extra batter, make it into pancakes) and do not open the oven to rotate the pans during baking.
Bake until the tops are puffed and a muffin bounces back when you poke it gently in the center with a finger, 18 to 22 minutes for standard muffins or 25 to 28 minutes for jumbo. Because these muffins are dark in color, it’s a little more difficult to tell when they’re done. If you’re not sure, then slip a small sharp knife or a metal skewer into the center of a muffin; if it comes out clean, then the muffins are done.
Remove the muffins from the oven and let cool in the pan for 5 to 10 minutes. Carefully lift the muffins from the pan and transfer them to a wire rack to cool a little more. (Use a butter knife to lift the muffins out if you didn’t use paper liners.)
Dust the tops with confectioners’ sugar just before serving, if desired. Serve warm.