Mix butter and sugar till fluffy.
Add egg, vanilla extract and mix briefly.
In a separate bowl, mix the flour, cocoa powder and salt.
Alternately add the dry ingredients and the milk until everything is blended.
Line a baking tray with baking paper. Put the batter into a piping bag, snip the bottom edge off and pipe onto the baking tray in small circles of 4 cm, leaving some space around each, since they run out a bit during the baking. Bake at 150°C for about 10 to 12 minutes till done.
Beat the coconut filling and the milk at high speed till smooth. Add the coconut slivers and mix briefly.
Pipe a thick layer of filling onto the lower part of the whoopie and place the upper part on top. Press lightly and roll the sides through the coconut slivers.