Line a 12-section bun tray with paper cake cases. Put the butter, sugar, eggs, flour, baking powder and vanilla extract into a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Stir in the polka dots and then divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Working with one colour at a time, roll out the icing on a work surface lightly dusted with icing sugar and cut out small letter shapes using alphabet cutters. Transfer to a tray lined with baking parchment. Re-roll the trimmings to make more. Leave to set for at least 1 hour until dry and holding their shape.
Spread each cake with a thin layer of melted chocolate and scatter plenty of letters on top, if necessary securing them to one another with a dampened paintbrush.