Whoopies

Preheat oven to 375@. Cut 6 pieces of foil to fit cookie sheets. Sift
together the flour, baking powder, baking soda, cream of tartar and salt
and set aside. In the large bowl of an electric mixer, cream the butter.
Add the vanilla and sugar and beat to mix well. Add the eggs, one at a
time, beating well after each addition. Scrape bowl with a spatula as
needed. On low speed add the cocoa and then the sifted dry ingredients,
alternating with the milk, in 2 additions, beating only til smooth after
each addition. It is important for these cookies to be shaped evenly and as
close to the same size as possible. Fill a 1/4 cup measuring cup with
dough. Level it off and transfer dough to the foil. Shape each mound of
dough as round as possible. Place 5 mounds of dough on 1 piece of foil.
Slide a cookie sheet under each piece of foil and bake for 20 mins. The
cookies are done when they spring back very quickly and surely when lightly
pressed in he center with a fingertip. Slide the foil off the sheet and let
stand for a minute or two. Then with a metal spatula, loosen the cookies
and carefully transfer them to large racks to cool. You will have 27 very
large cookies. If they are not all the same size, pick out the ones the
match each other the most closely and form them into pairs. There will be
one left over. Now lay them out in pairs, opened, with the flat sides up.
FOR FILLING: Place the flour in a 1 quart saucepan. Add the milk gradually,
stirring with a rubber spatula. If the mixture is not smooth, strain it
before cooking. Place it over moderate heat. Cook, stirring and scraping
the bottom constantly with rubber spatula, til mixture becomes very thick
and bubbles slightly. Simmer, stirring, for about 2 mins. If necessary,
beat with a small wire whisk to make the mixture smooth. Stir in 1 tbsp of
the butter and set the mixture aside to cool at room temp. Stir
occasionally while cooling. Place the remaining butter in the large bowl of
an electric mixer and beat til it is slightly softened. Gradually add the
salt, vanilla and sugar and beat well for about 2 mins, scraping bowl as
necessary with spatula. Gradually add the cooled flour mixture. Then beat
at high speed for a minute or two til the filling is smooth, light and
fluffy. TO ASSEMBLE: Place a generous heaping tablespoonful of the filling
on the center of one of each pair of cookies. With the back of a spoon
spread the filling out to about 1/2″ from the edges. Top with another
cookie, flat side down. Press down gently on the top cookie. Press all
around to spread filling almost to the edges. Store in the refrigerator or
freezer (in both cases, put wax paper between the cookies to prevent
sticking).