I usually use condensed milk or pureed fruits and milk depending on the kind of cake I am making. To make a sponge cake I use condensed milk and then add soda bi carb and baking powder to leaven the cake. In a recipe for cake using eggs very little leavening is required. The recipe for an eggless cake would differ drastically from one that uses eggs. Most often you cannot substitute eggs directly with another substitute in a specific proportion, the sugar balance and the liquid levels have to be adjusted with milk or water.