Line a large baking sheet with baking paper. Place the flour, cocoa, baking soda and light brown sugar in a large bowl. Mix the egg, oil and buttermilk with the boiling water and stir into the dry ingredients until well mixed. Spoon about 30 tablespoonfuls on to the tray, making sure they are well spaced, and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes or until just firm. Leave to cool.
Beat together the cream cheese and peanut butter until smooth. Sift in the icing sugar and beat until well combined. Pipe or spoon over the icing over 15 halves, then sandwich the remaining 15 on top.
Melt the chocolate over a small bowl set in a pan of simmering water, making sure the water does not touch the bottom of the bowl. Drizzle over the whoopie pies before serving.