Preheat oven to 375°F.
In a large bowl, beat the butter and sugar for a minute or so, until well combined. It will have the consistency of wet sand. Add the egg and vanilla and beat until smooth.
In a medium bowl, stir together the flour, cocoa and salt. In a small bowl, stir the baking soda into 1/2 cup very hot water until dissolved.
With the mixer on low or stirring by hand, add about a third of the dry ingredients, then half the buttermilk and half the baking soda mixture, mixing each time just until blended. Repeat with another third of the dry ingredients, the rest of the buttermilk and baking soda mixture, and then the rest of the dry ingredients, mixing each time just until combined. Stir in the chocolate chips. The batter will be quite wet, almost like cake batter. (You want them to be cakey and soft, so that the filling doesn't squish out the end when you bite into it.)
Drop large, round spoonfuls of batter 2 inches apart on a cookie sheet that has been sprayed with nonstick spray. (If you’re making pies, try to keep the mounds similar in size and shape so they will make even sandwiches). Bake for 12–15 minutes, until the tops no longer appear wet and just spring back when lightly touched. Transfer to a wire rack to cool.
Meanwhile, to make the frosting, combine the sugar, corn syrup, water, and egg whites in the top of a double boiler or in a clean stainless steel bowl set over a pot of simmering water. Make sure the simmering water doesn’t touch the bottom of the bowl or double boiler – you only need an inch or two of water in the pot. Beat the sugar mixture with an electric mixer on high speed for about 7 minutes, until it stands in billowy peaks. Remove it from the heat and beat in the vanilla.
When the cookies are completely cool, spread the bottom of half the cookies with frosting or marshmallow cream, and sandwich with a second cookie. Store extras individually wrapped in plastic wrap. Makes about 1 dozen whoopie pies.
Serving Size Makes 1 dozen pies or 2 dozen cookies.