1.Preheat oven to 120°C. Grease a 24cm x 32cm swiss roll pan; line base and long sides with baking paper, extending paper 5cm over sides.
2.Stir butter, sugar and syrup in medium saucepan over low heat until sugar dissolves. Remove from heat. Stir in the combined water and soda. Stir in coconut, oats and sifted flour. Spread mixture into pan.
3.Bake slice about 45 minutes. Stand slice in pan 15 minutes before transferring to wire rack to cool.
4.Meanwhile, make golden icing.
5.Spread slice with icing; stand at room temperature until set before cutting.
6.Sift icing sugar into medium heatproof bowl; stir in syrup, butter and enough of the water to make a thick paste. Set bowl over medium saucepan of simmering water; stir until icing is spreadable.
If you want the slice to be fudgy in texture, decrease the baking time by 5-10 minutes.