1-First Step: Preheat the Oven and Prepare Water Bath. Preheat your oven to 320°F (160°C). Prepare a bain-marie by filling a baking tray with hot water. This water bath helps regulate baking temperature and humidity, preventing cracks and ensuring even cooking.

2-Second Step: Separate Eggs and Prepare Yolk Batter. Carefully separate the egg yolks from the whites to avoid contamination. In a pot over low heat, gently melt butter, cream cheese, and ¼ cup (50g) sugar, stirring until smooth. Remove from heat and whisk in the egg yolks one at a time until fully incorporated.

3-Third Step: Add Dry Ingredients and Milk. Sift cornstarch and flour into the yolk mixture, stirring gently to blend until smooth and lump-free. Add the milk and any optional flavorings like vanilla or lemon juice, mixing well to create a smooth, slightly liquid batter.

4-Fourth Step: Beat Egg Whites to Meringue. In a clean bowl, beat egg whites with cream of tartar starting at a low speed until foamy. Gradually add ¼ cup (50g) sugar a tablespoon at a time, increasing speed until stiff but not overly stiff peaks form. Avoid overbeating to prevent deflation.

5-Fifth Step: Fold the Meringue into the Batter. Gently fold about one-third of the meringue into the yolk batter to lighten it. Then carefully fold in the remaining meringue, cutting through the center and sweeping around the bowl, lifting the mixture to retain airiness without deflation. This step is key to the jiggly texture.

6-Sixth Step: Bake the Cheesecake. Line an 8-inch cake pan with parchment paper. Pour the batter into the pan and place it in the water bath inside the preheated oven. Bake for 30 minutes with the door closed, then 20 minutes with the door slightly ajar, followed by 20 minutes with the door closed again. Adjust baking times and oven temperature (up to 330°F/165°C) if the cake browns too quickly or fails to rise.

7-Final Step: Cool and Serve. The cheesecake is done when the top is golden-brown and the center has a slight jiggle but feels springy. Remove from oven and let rest inside for 10 minutes. Carefully invert onto a plate for a smooth top. Serve warm for maximum fluffiness and jiggle, or chill for a firmer chiffon-like texture. Store leftovers in an airtight container in the refrigerator for 2-3 days; freezing is not recommended.