Place milk and butter into a small bowl, heat under medium low heat in microwave for 1 minute, mix a few times until butter is melt, set aside to chill down.
Preheat oven to 170 degrees C (338F). Grease butter on baking pan, then sprinkle with flour.
In a large bowl, add egg white and mix by medium speed with a electronic mixer until it shapes a very light, fluffy and even foam (left picture). Add half amount of sugar and keep mix by medium speed until sugar is cooperate, then add the rest of sugar, keep mix until the texture turns to creamy and firm. When you pull the mixer out off the mixture, the cream could shape a triangle and stand. Then it's ready. (right picture)
Add egg york and mix by low speed for a few seconds, then add vanilla extract and mix for another few seconds.
Switch from electronic mixer into rubber spatula. Whisk flour into the mixture and use spatula to mix gently, until flour is fully cooperate. Do not over mix. Slowly add milk and butter, and mix gently until the mixture turns to a even and medium thick batter. Pour batter into baking pan.
Place 2 layers of towel on table, drop the baking pan from 20 centimeters (0.6 feet) high, in order to get the surface of batter flat and get extra air out. Place in low level of oven and bake for around 30 minutes, until the surface turns brown. Put a toothpick into the cake and pull out, the toothpick should come out clear.
After the cake is cooked, take out from oven, and immediately drop the baking pan from 30 centimeters (1 feet) high. Flip baking pan on a clean wet towel, let it rest for 2 to 3 minutes, in order to flat the surface. Flip the cake again and place it on a cooling rack, cover with a clean wet towel in order to keep the cake moist.
After the cake is cooled, slice it into half. To make the slice accurate and straight, use several toothpicks to line up the half way of cake, use a bread knife to slice against toothpicks. Set aside for decoration.