Wrap a 15cm square mold with two layers of aluminum foil to prevent water from seeping in, then line it with parchment paper and set aside. Preheat the oven to 150°C and prepare a larger pan for a water bath.

Separate 4 medium eggs at room temperature. Warm 50g vegetable oil and 70g milk in the microwave, then sift in 60g cake flour and whisk until smooth.

Add the egg yolks one at a time to the flour mixture, mixing thoroughly after each addition. Stir in 1 tsp vanilla extract for extra flavor.

Whip the egg whites with 60g granulated sugar using an electric mixer with a whisk attachment until it reaches the right consistency. Gradually fold the whites into the egg yolk mixture in three batches to maintain the batter’s fluffiness.

Pour the batter into the prepared mold, smooth the surface, and tap the mold to release air bubbles. Place the mold in a hot water bath (80°C) and bake for 45 minutes.

Once baked, remove the bouncy Castella cake from the mold, peel off the baking paper, and cool on a wire rack. Cut the cooled cake into four slices.

Whip heavy cream with sugar (optional) until creamy, then transfer to a piping bag. Pipe the cream between the cake slices and serve with milk for a perfect treat.