I love this recipe because it isn't too sweet.
Golden crumbs give way to tender crumb structure and tart pockets of baked fruit in this traditional pastry. Achieving the correct mouthfeel relies entirely on physical leavening rather than chemical agents alone. Separating the eggs and whipping the whites introduces vital air pockets that expand steadily in a 350-degree environment. Folding those aerated whites gently into the dense flour mixture prevents the collapse of delicate micro-bubbles while maintaining an exceptionally light pastry texture. Creaming the margarine and sugar thoroughly incorporates microscopic air pockets that fat molecules trap, forming the foundational scaffolding necessary for a high rise without a heavy, dense crumb.
Integrating dry components alternately with skim milk prevents gluten strands from overdeveloping, which would otherwise result in a tough, rubbery cake instead of a tender confection. Overmixing releases too much gluten and destroys the aeration achieved by the whipped egg whites. When arranging halved ripe plums over the batter surface, press them lightly without submerging them entirely so moisture evaporates evenly during baking without creating soggy pockets.