Preheat the oven to 350°F. Lightly grease (or line with parchment) two baking sheets.
To make the cakes: In a large mixing bowl, beat together the butter, sugar, espresso powder, baking powder, baking soda, salt, and vanilla till smooth. Add the egg, again beating till smooth.
Add the cocoa, stirring to combine.
Add the flour to the batter alternately with the milk, beating till smooth. Scrape down the sides and bottom of the bowl, and beat again briefly to soften and combine any chunky scrapings.
Drop the dough by the 1/4-cupful onto the prepared baking sheets, leaving plenty of room between the cakes; they'll spread. A muffin scoop works well here.
Bake the cakes for 15 to 16 minutes, till they're set and firm to the touch. Remove them from the oven, and cool on the pans. While still lukewarm, use a spatula to separate them from the pan or parchment; then allow to cool completely.
**To make 16 smaller whoopie pies (about 2 3/4" dia., about 2 1/2 ounces each): drop the dough onto the baking sheets in 30g balls, about the size of a large ping pong ball, or about 1 1/4". Bake for 11 to 12 minutes, till set. Finish as directed in the recipe.
To make the filling: Beat together the shortening, sugar, and marshmallow till well combined.
Dissolve the salt in the water, and add to the marshmallow mixture. Add the vanilla, and beat till smooth.
Spread the flat side of half the cakes with the filling. Top with the remaining cakes, flat side towards the filling. Wrap individually, in plastic wrap, till ready to serve.
Yield: 8 large whoopie pies (about 4" dia., about 5 ounces each). To make smaller pies, see ** in step 7.
Due to the differences between natural and Dutch-processed cocoa powders, do not substitute one for the other in recipes.
Dutch Process cocoa has been treated with an alkali to neutralize its natural acidity. Because it is neutral and does not react with baking soda, it must be used in recipes calling for baking powder, unless there are other acidic ingredients in sufficient quantities used. It has a reddish-brown color, mild flavor, and is easy to dissolve in liquids.
Espresso powder is chocolate's best friend. Use 1/2 to 2 teaspoons in chocolate baked goods, frostings, and sauces; a touch of espresso powder enhances chocolate's flavor without adding any coffee flavor of its own.
Ground, brewed, then dried from specially selected coffee beans, powder readily dissolves for easy mixing.
*This ingredient is optional.
Marshmallow Fluff is a New England ingredient available in certain other parts of the country. Marshmallow creme, more widely available, is a thinner consistency than Fluff. If you make your filling with marshmallow creme, you may need to add a bit more sugar to stiffen it up sufficiently.