In a medium mixing bowl, whisk together the flour, spices, baking soda, and salt.
In a separate large bowl, combine the brown sugar, melted butter, molasses and crystallized ginger. Whisk until combined. Add the egg and buttermilk and continue to whisk until blended.
Add the flour mixture and fold together using a rubber spatula. Do not over mix.
Cover the bowl with plastic wrap and refrigerate for 1 hour or overnight.
Preheat oven to 350° F. Line a baking sheet with parchment paper or a silpat mat. Using a 1 tablespoon cookie scoop, roll the dough into a ball and place 2-inches apart on the cold baking sheet. Bake until the cookies are puffy, about 11-12 minutes. Let the cookies cool in the baking sheet for 5 minutes before transferring to a wire rack to cool.
Repeat with the remaining dough using a cold baking sheet each time.
To prepare the filling, beat together the 6 tablespoons butter with 1 cup of powdered sugar. Once smooth and blended, add the remaining 1 cup of powdered sugar and mix until incorporated. With the mixer running on medium, add the cream cheese one piece at a time then add the vanilla and lemon zest. Pipe onto half the cookies and top with a matching size cookie, bottom side down.
Sprinkle with powdered sugar and serve.