Dust a 23cm square dish generously with icing sugar.
In a small saucepan over medium-high heat, stir together caster sugar, golden syrup and 175ml water. Heat to about 125 degrees C, or until a small amount of syrup dropped into cold water forms a rigid ball.
While syrup is heating, place remaining water in a metal bowl and sprinkle gelatine over the surface. Place bowl over simmering water until gelatine has dissolved completely. Keep in a warm place until syrup has come to temperature. Remove syrup from heat and whisk gelatine mixture into hot syrup. Set aside.
In a separate large mixing bowl, whip egg whites to soft peaks. Continue to beat, pouring syrup mixture into egg whites in a thin stream, until the egg whites are very stiff. Stir in vanilla. Spread evenly in prepared dish and let rest 8 hours or overnight before cutting.