Step 1: Prepare the Sponge Cake Batter

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a 10×15-inch jelly roll pan with parchment paper. Lightly grease the paper to ensure the cake comes out smoothly.
2. Whisk the Eggs: In a large mixing bowl, beat the eggs and sugar together using an electric mixer. Start on a low speed and gradually increase to medium-high. Beat for about 5–7 minutes, or until the mixture becomes thick and pale. This is the key to getting that airy texture in the sponge.
3. Add Vanilla: Once the eggs and sugar have reached the desired consistency, add the vanilla extract and mix until combined.
4. Sift the Dry Ingredients: In a separate bowl, sift the cake flour, cornstarch, baking powder, and salt together. Sifting ensures that the dry ingredients are evenly distributed and helps prevent lumps in the batter.
5. Incorporate Dry Ingredients: Gently fold the dry ingredients into the egg mixture using a spatula. Be careful not to deflate the air bubbles you’ve created by overmixing.
6. Add Butter and Milk: Warm the milk and melt the butter. Gently fold both into the batter, ensuring everything is well combined.

Step 2: Bake the Cake

1. Pour the Batter: Pour the cake batter into the prepared jelly roll pan. Use a spatula to spread the batter evenly.
2. Bake: Place the pan in the preheated oven and bake for 10–12 minutes, or until the cake springs back when touched and a toothpick inserted into the center comes out clean.
3. Cool the Cake: Once the cake is done, remove it from the oven and immediately run a knife around the edges to loosen it. Place a clean kitchen towel over the cake and flip it onto a flat surface. Carefully peel off the parchment paper. Let the cake cool for 10 minutes.

Step 3: Prepare the Filling

1. Whip the Cream: While the cake is cooling, use an electric mixer to whip the heavy cream until soft peaks form. Gradually add powdered sugar and vanilla extract, continuing to beat until stiff peaks form.
2. Flavor Variations: At this stage, you can add additional flavorings. For instance, mix in a bit of matcha powder for a Japanese twist or fresh fruit like strawberries for a refreshing taste.

Step 4: Roll the Cake

1. Fill the Cake: Once the cake has cooled, place a piece of parchment paper or a kitchen towel under the cake. Spread a thin layer of the whipped cream filling evenly across the cake, leaving a small border around the edges. Add fruit or other fillings if desired.
2. Roll Carefully: Starting from one end, gently roll the cake up tightly, making sure to roll evenly to avoid cracks. If the cake cracks a little, don’t worry—it’ll still taste delicious!
3. Chill: Once rolled, wrap the cake in the parchment paper or plastic wrap and refrigerate for at least 30 minutes to allow the filling to set and the roll to firm up.

Step 5: Slice and Serve

1. Slice the Cake: After the cake has chilled, slice it into neat rounds. You should see a beautiful spiral of cream inside.
2. Serve: Arrange the slices on a plate and serve with a dusting of powdered sugar or a drizzle of chocolate sauce for added flair.