To make the boiled icing, put the sugar in a saucepan, add 150ml water and clip a sugar thermometer on to the side of the pan. Set over a low heat until the sugar has dissolved. Add the glucose syrup. Bring to the boil and, when it gets very close to 116C, start whisking the egg whites to stiff peaks using an electric whisk. As soon as the syrup reaches 116C, continue to whisk as you pour a thin stream of syrup into the egg whites. The mixture will billow up into a thick, opaque meringue and then collapse down again slightly. Whisk in the vanilla, and after a couple of minutes pour the icing over the cake. Don’t leave too long as it thickens as it cools. Using a wet palette knife, smooth the icing over the cake.