Melt the chocolate together with the butter (see
Break the eggs into a mixing bowl, then add the sugar and sift in the flour. Stir them together vigorously.
Stir the melted chocolate and butter and carefully pour them into the mixing bowl. Stir the mixture until it is smooth, then pour into a shallow cake tin, 20 cm (8 inches) square, lined with nonstick baking paper, using the spatula to scrape every last bit from the bowl. Place on the top shelf of a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until just firm when you touch it very gently in the middle.
Allow the cake to cool in the tin, then cut it into 9 pieces in the tin. Using the icing pens, decorate the squares with icing scribbles, if liked.