Preheat the oven to 170°C / fan oven 150°C /325°F, Gas Mark 3. Lightly grease baking sheets.
Sieve together the cocoa, flour, baking soda, baking powder, and salt.
Lightly beat together the milk and vanilla extract.
Add the cocoa and flour mixture to the butter mix, alternating with the milk mixture; beating until smooth. Fold in the chocolate chips.
Spoon the batter into mounds onto the baking sheet flattening the batter lightly with the back of a spoon. Make sure you leave enough room for the mixture to spread. This mixture should make about 12 cakes – that means 6 whole whoopies when filled. You may have to do this in two batches.
Cook for 15 minutes or until they are firm to the touch. Remove from oven and let cool completely on a wire rack.
For the filling:
Melt the white chocolate in the microwave on High for 1½ minute, stirring halfway. Leave to cool. Beat the Stork and icing sugar in a large bowl until creamy. Beat in the chocolate. Cover and chill until ready to use. Spread the filling on half the whoopies and sandwich with another whoopie.
For the topping:
Place the icing sugar and cocoa powder in a bowl, drizzle in about the quarter of the water and stir together. The icing should run off in thick ribbons. If not, add a tiny little more water, but don't overdo it. Drizzle the icing over the whoppies and decorate shards of white chocolate.