Sieve together the flour, baking powder and salt. Set aside.
Stir together the sugar and egg yolks until thick and creamy. Add melted butter. Add flour mixture and gently stir to combine. Add milk 1 tablespoon at a time, stirring after each addition, just until the cake mixture drops easily from the spoon.
Beat the egg whites to stiff peaks. Fold into the previous mixture. Turn into a well greased 34x28cm tin.
Bake in a preheated 200 C / Gas 6 oven for approx 20 mins. Let cool in tin, then turn out on to a clean work surface and cut in half to make two layers. Spread one half with jam, then replace the top half. Cut into slices and arrange on a platter. Dust with icing sugar if desired.