You can use baking soda and vinegar instead (Ukrainians and Russians use this mixture more often than baking powder). Add ½ tsp baking soda and ½ tsp white vinegar (or lemon juice) straight to the batter (don’t be afraid if your batter will look like a small volcano, it’s normal – that’s how carbon dioxide is formed), but the reaction won’t last forever, so once both vinegar (or lemon juice) and baking soda are added to the batter, stir well and quickly put this apple sponge cake in the oven.