Bolu kukus — Indonesia

Bolu kukus is a steamed sponge cupcake: a wheat-flour batter beaten with eggs, sugar, and soda water instead of a chemical raising agent alone, then cooked in a steamer instead of an oven until the top splits open into the cracked pattern the cake is known by. It is reported across Indonesia as an everyday snack cake, sold from market stalls and home kitchens alike, and is grouped within the wider kue bolu family of Indonesian sponge cakes, which the word bolu itself, borrowed from the Portuguese bolo, points back to. The batter is built by beating eggs with sugar until pale and thick, folding in sifted wheat flour, and finishing with a carbonated soft drink, most often a lemon-lime soda, which gives the crumb its lift and helps crack the surface as it steams. Vanilla, cocoa, pandan, or strawberry flavoring is folded through the base batter, and the mixture is poured into paper cups set in a steamer basket over vigorously boiling water so the burst of heat forces the batter to split open in the first minutes of cooking. Set beside oven-baked cupcakes of the same batter family, this version is defined chiefly by the steaming method itself: the paper cups sit directly over steam rather than dry heat, which keeps the crumb noticeably moist and soft and produces the split, domed top that gives the cake its name and its usual presentation.