Preheat oven to 350°F. Put the eggs and the lemon zest in the bowl of an electric mixer and using the paddle attachment, beat on medium speed until light and frothy. Add the molasses and beat for 1 minute.
Mix the flour with the dry spices.
Add the brown sugar to the egg mixture and beat until well blended, about 2 minutes.
With the mixer on low speed add a third of the flour, then the melted butter, then another third of the flour and mix just until blended.
In a small bowl, stir the baking soda and the buttermilk together. Add this to the mixer bowl with the egg, sugar and flour and mix just until blended.
Add the last of the flour just until combined.
Coat 10 muffin cups with nonstick spray or butter and flour them. Fill each cup to ¼ inch from the top.
Add 1 tablespoon of crystallized ginger to each muffin, pushing it down a bit into the batter.
Bake for 25 minutes until a toothpick inserted in the center comes out clean.
[*To chop crystallized ginger, use a food processor and add 1-2 tbsp sugar. Pulse gently, being careful not to overwork the machine.]