Separate 5 eggs into yolks and whites: Crack an egg into a small bowl. Using clean hands, gently scoop out the egg yolk and place it into the egg yolk bowl. Pour the remaining egg white into the egg white bowl. Repeat with the rest of the eggs. (This method helps ensure no yolk breaks and contaminates the egg whites, which is crucial for whipping them properly.)
In a large bowl, combine 40 g vegetable oil, 5 egg yolks, 70 g milk, and ½ tsp vanilla extract. Whisk until well mixed.
Sift in 85 g cake flour, and 1 g baking powder (optional). Whisk until smooth. Set aside.
Brush a very thin layer of vegetable oil inside the rice cooker. Close the lid and set it to the "Warm" setting.
Add ½ tsp lemon juice (optional) and the first 25 g of sugar to the egg whites.
Using a hand mixer on medium-low speed, whip until the volume doubles.
Add the next 25 g sugar, whip on medium speed until the egg whites turn from translucent to white.
Add the final 25 g sugar, whip until stiff peaks form. (*See pictures above for the difference between soft and stiff peaks.)
Scoop ⅓ of the whipped egg whites into the egg yolk mixture and fold gently until combined.
Transfer the yolk mixture back into the remaining egg whites. Fold gently until the color is even.
Pour the batter into the preheated rice cooker.
Gently shake the rice cooker to even out the batter surface. ⚠️ Be careful—it’s hot, so wear oven mitts.
Cover and cook on the "White Rice" setting for 15 minutes.
Do not open the lid. Switch to "Warm" for 10 minutes. This helps the cake hold its shape before testing.
Open the lid and insert a bamboo skewer to the center of the cake. If it comes out clean, the cake is done.
If batter sticks, close the lid and cook for 5 minutes, then warm for 5 minutes. Repeat the skewer test until it comes out clean. (⚠️ Cooking times vary by rice cooker. Overcooking may cause the cake to shrink.)
Once the bamboo skewer comes out clean, carefully remove the cake from the rice cooker by flipping it onto a large plate or a cooling rack.
You can enjoy the cake warm directly for a soft, fluffy treat. Or let it cool completely, then slice it to use as a birthday cake base. Spread whipped cream on top or decorate with fresh fruits for a beautiful presentation.