Passion Fruit Sponge

From 1922:

Three eggs, 3 tablespoonfuls sugar, half cupful flour, 1 cupful icing sugar, 2 passion fruit, 1 oz. butter. Beat the eggs and sugar for 20 minutes very lightly, add flour; bake in sandwich tins; turn out, and let cool. Sift icing sugar, add to it the butter and passion fruit, and then melt with about teaspoonful of boiling water. When sponge is quite cool, not before, spread gently with fruit, icing one, place the other on top, and spread that; set for a short time before cutting it. Note: The hot, or better still, boiling water to melt icing sugar gives the icing a glossy appearance when set.