Heat the oven to 350°F (175°C) and line baking sheets with parchment. Set the racks so the trays can bake evenly, working in batches if necessary.

Cream the softened butter and packed light brown sugar for 2 to 3 minutes, until light and fluffy. Beat in the egg, then mix in the molasses and water; the mixture may look slightly curdled.

In a separate bowl, whisk together the all-purpose flour, ground ginger, cinnamon, baking soda, and salt. Add the dry mixture to the wet mixture in batches, mixing only until combined.

Drop rounded tablespoons of batter onto the prepared sheets, leaving about 2 inches between portions. A small cookie scoop helps create matching cakes that bake at the same rate.

Bake for 10 to 12 minutes, until the cakes are puffed and the tops spring back lightly when touched. Do not wait for the cakes to look dry, because overbaking will reduce their soft texture.

Cool the cakes on the baking sheet for about 5 minutes, then transfer them to a rack. Let them cool completely before adding the filling.

Beat the softened butter and shortening until smooth and fluffy. Gradually beat in the sifted confectioners' sugar, then add the marshmallow creme and vanilla and mix until creamy.

Pipe or spread marshmallow buttercream onto the flat side of half of the cooled cakes. Top with the remaining cakes and press gently so the filling reaches the edges without squeezing out.