For the gingerbread: Preheat the oven to 350 degrees F.
Line baking sheets with parchment paper and lightly spray with nonstick baking spray.
Whisk together the flour, salt, baking soda, ground ginger, cinnamon, cloves, and nutmeg in a small bowl and set aside.
Combine the butter, oil, and sugars in the bowl of a stand mixer and beat using the paddle attachment and until light and fluffy. Add the egg and fresh ginger and beat until combined.
Combine the molasses and buttermilk in a measuring glass. Add the flour mixture and buttermilk/molasses mixture alternately in 2 batches, starting and ending with the flour, and mix until just combined. Remove the bowl from the stand and fold in the crystallized ginger. Cover and refrigerate the batter for 20 minutes.
Using a small ice cream scoop, scoop 6 mounds batter onto a baking sheet, spacing the mounds about 3 inches apart. Bake until the cakes just spring back when touched (should be ever so slightly under-baked because the cakes will continue to bake on the hot baking sheets), about 10 minutes. Remove to a baking rack and let sit on the sheet pan for 5 minutes. Remove from the baking sheets and let cool completely on a baking rack, about 10 minutes.
For the filling: Bring a medium pot with 1-inch water and bring to a simmer over medium-high heat. Reduce the heat so that the water is at a bare simmer.
In the bowl of an electric mixer, combine the egg whites, lemon juice, water, sugar, zest, cream of tartar and salt and whisk until combined and slightly foamy, about 1 minute.
Place the bowl over the simmering water and stir with a rubber spatula until it reaches 160 degrees F, about 2 minutes. Remove the bowl from the water and place it on the mixer stand. Using the whisk attachment, beat the mixture at high speed until it forms, stiff peaks and is cool, about 7 minutes. Add the vanilla extract and beat for 10 seconds longer. Remove from the stand and gently fold in the lemon curd then molasses, leaving streaks of curd and molasses in the filling.
Place a large dollop of the filling on the center of the flat side of half the cakes. Top with the flat side of the remaining cakes and gently press until the filling spreads to the edge of the cake.
Combine lemon juice, sugar and salt in a small saucepan and bring to a boil over high heat. Whisk together the eggs in a small bowl and temper in the lemon juice. Return mixture to the pan and cook over medium heat, stirring with a wooden spoon, until the mixture thickens and coats the back for a spoon.
Remove from heat, whisk in the cold better and strain into a bowl. Stir in the zest, cover and refrigerate until firm and cold, about 2 hours.