Lightly grease a 12-hole muffin tin or line it with paper cases.
Place the cocoa powder in a large bowl, pour in the boiling water and coffee essence and stir until smooth. Add the melted butter and stir well.
Whisk the eggs and sugar into the cocoa mixture, then add the prunes and sift in the flour. Gently fold together.
Spoon the mixture carefully into the prepared muffin tin and bake in a preheated oven, 190˚C (375˚F), Gas Mark 5, for 10–12 minutes, until risen and firm to the touch. Transfer to a wire rack and serve warm or cold with whipped cream, if desired.