Grease an 8-inch round cake pan (or a similar heat-proof dish) and line the bottom with parchment paper. Prepare your steamer: fill the bottom pot with water (about 2-3 inches deep) and bring it to a boil over medium-high heat.

In a large mixing bowl, combine the room-temperature eggs and granulated sugar. Using an electric hand mixer, beat on high speed for 8-10 minutes, or until the mixture is very pale yellow, thick, and fluffy, almost like a ribbon when lifted. This step is crucial for the cake's texture.

In a separate small bowl, sift together the all-purpose flour and baking powder.

Gently fold the sifted flour mixture into the egg mixture in three additions using a spatula. Be careful not to deflate the eggs; fold in a circular motion, cutting through the center and bringing the batter up from the bottom.

In a small bowl, whisk together the milk, vegetable oil, and vanilla extract (if using). Pour this wet mixture over the cake batter and gently fold until just combined. Do not overmix.

Pour the batter into the prepared cake pan. Gently tap the pan on the counter a few times to release any large air bubbles.

Once the water in the steamer is boiling, carefully place the cake pan onto the steaming rack. Cover the steamer with its lid (wrap the lid with a towel to prevent condensation from dripping onto the cake) and steam for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean.

Once cooked, carefully remove the cake pan from the steamer. Let it cool in the pan for 5-10 minutes before inverting it onto a wire rack to cool completely. Slice and serve warm or at room temperature.