Put the sugar, glucose syrup and water to a thick-bottomed saucepan and boil until the mixture reaches a constant temperature of 100 °C. Soften the gelatine sheets in plenty of water for 5 minutes. Strain off the water and melt the gelatine sheets. Beat together the egg whites, icing sugar and food colouring (if using) in a bowl until thick, using an electric whisk. When the mixture in the saucepan reaches 127 °C, pour it into the egg white mixture, beating constantly. Add the melted gelatine and continue whisking until the mixture is stiff and shiny and has started to cool. Mix together the icing sugar and potato starch, and sprinkle over the base of a large oven pan. Put the mixture into a piping bag and pipe out lengths in the oven pan. Leave to set at room temperature. Cut the lengths into pieces of the desired size. Roll the pieces in the icing sugar mixture. Store in a container at room temperature.
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