Preheat the oven to 350°F and line a large baking sheet pan with parchment paper.
Whisk together the all-purpose flour, cocoa powder, baking soda, cinnamon, salt, and nutmeg in a large bowl. Set aside.
Cream together the butter and sugars in a large bowl or stand mixer fitted with a paddle attachment until light and fluffy, about 1 to 2 minutes.
Add the eggs, heavy cream, chocolate syrup, and vanilla extract to the butter and sugar mixture. Beat on medium speed until well combined, about 1 additional minute.
With the mixer on low speed, add the flour mixture to the wet ingredients and beat until completely combined.
Using a 2 Tablespoon medium cookie scoop or spoon to drop batches of batter on the sheet pan, keeping them 2 inches apart. Bake in the oven for 16 to 18 minutes.
In the meantime, use the mixer to beat together the marshmallow fluff, 1 cup powdered sugar, butter, vanilla, and lemon juice. Beat for several minutes until light and fluffy. Gradually incorporate the rest of the powdered sugar as needed to allow the filling to stiffen and set.
Chill the marshmallow mixture for at least 10 minutes in the refrigerator.
Let the cakes completely cool before assembling your whoopie pies.
To assemble your whoopie pie, take two cakes and spread a layer of filling on the inside of one cake (leaving some space at the outer edge) then place the top cake on the filling creating a sandwich and lightly press down.
Chill until ready to plate and serve.