Preheat the oven to 180ºC/gas 4. In a food processor, blitz the oats to a fine powder. Roughly chop the chocolate. Add the oats and chocolate to a bowl with the flour and baking powder, mix to combine and set aside.
Place the butter, peanut butter, sugars and vanilla extract in a bowl and, using either an electric whisk or by hand, beat them together until whipped and creamy.
Add the eggs, followed by the dry ingredients, and mix well with a wooden spoon until everything is combined and you have a stiff dough.
Place heaped teaspoons of the mixture on the baking sheets, leaving 3cm between each one. Bake in the oven for 8 to 10 minutes, until golden around the edges, then remove from the oven and leave to cool for 5 minutes before transferring to a wire rack to cool completely.
To make the icing, sift the icing sugar into a bowl and mix in a few drops of water until it’s a spreadable consistency. You could add natural food colouring, if you like.
Ice the cooled cookies and decorate with Smarties (if using). Get the kids to write names on them with icing pens, or make a little extra icing in a different colour, and spoon it into a small piping bag with a fine nozzle.