Preheat oven to 190 degrees C (gas mark 5). Grease and flour a 20x30cm rectangular baking tin.
Sift together the flour, baking powder and salt. Set aside.
In a large bowl, cream together the butter, sugar and vanilla until light and fluffy. Add the eggs one at a time, beating well with each addition. Add the flour mixture alternately with the milk; beat well.
Pour this mixture into the tin. Bake in preheated oven for 30 to 40 minutes, or until a skewer inserted into the centre of the cake comes out clean. Let stand for 5 minutes, then turn out onto a wire rack and cool completely. Store overnight to give the cake a chance to firm up before icing.
To make the icing: In a large bowl, combine icing sugar and cocoa. In a saucepan, heat milk and 1 tablespoon butter until the butter is melted. Add the milk to the sugar mixture and mix well to create a fluid, but not too runny, icing.
Procedure: Cut the cake into 24 squares. Place coconut in a shallow container. Using a fork, dip each square into the icing, then roll it in the coconut. Place onto a cooling rack to dry. Continue for each piece. The icing will drip, so place a sheet of greaseproof paper under the rack to catch the drips.