Preheat the oven to 350°F/180°C. Line 2 baking sheets with parchment paper. Put the egg, light brown sugar, and molasses in a mixing bowl and beat together with an electric handheld whisk until thickened and foamy. Beat in the lightly salted butter and milk.
Sift the flour, baking soda, ginger, and allspice into the bowl and stir with a wooden spoon to make a soft paste.
Spoon teaspoons of the mixture onto the baking sheets, flattening them slightly so each spoonful is about 1¼ inches/3 cm in diameter. Space the spoonfuls about 2 inches/5 cm apart to allow for expansion.
Bake in the preheated oven for 10 minutes, or until risen and firm to the touch, switching over the baking sheets halfway through baking. Let stand on the sheets for 5 minutes, then transfer to a wire rack to cool.
For the filling, put the cream cheese, unsalted butter, vanilla, and confectioners’ sugar in a mixing bowl and beat together with an electric handheld whisk until smooth and creamy. Beat in the boiling water to soften. Sandwich the whoopie pies together in pairs with the filling. Let stand in a cool place to firm up for a couple of hours, then dust with confectioners’ sugar.
This recipe is excerpted with permission from <a href="http://www.amazon.com/gp/product/1445452235/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1445452235&linkCode=as2&tag=craftfocom-20">"Mini Cakes: Irresistible Little Cakes, Bakes and Cake Pops,"</a> published by Parragon Books.