Step 1: Preheat your oven to 325°F (160°C). Line a 12-cup muffin tin with paper liners. Prepare a larger baking pan for a water bath.
Step 2: In a heatproof bowl set over a saucepan of simmering water (bain-marie), combine the softened cream cheese, milk, and butter. Whisk constantly until the mixture is smooth and fully melted. Remove from heat and let cool slightly.
Step 3: Add the egg yolks, lemon juice, vanilla extract, and a pinch of salt to the cream cheese mixture. Whisk until well combined and smooth.
Step 4: Sift together the all-purpose flour and cornstarch. Gradually add this dry mixture to the cream cheese batter, whisking until just combined and no lumps remain. Set aside.
Step 5: In a separate, clean, dry bowl, add the egg whites. Using an electric mixer, begin whipping the egg whites on medium speed until foamy.
Step 6: Gradually add the granulated sugar, a tablespoon at a time, while continuing to whip. Increase speed to medium-high. Whip until the meringue forms soft, glossy peaks. Do not overbeat to stiff peaks.
Step 7: Add about one-third of the meringue to the cream cheese batter. Gently fold it in with a spatula to lighten the batter.
Step 8: Add the remaining meringue in two additions, folding very gently until just combined. Be careful not to deflate the meringue.
Step 9: Carefully pour the batter evenly into the prepared muffin liners, filling each about three-quarters full.
Step 10: Place the muffin tin into the larger baking pan. Pour hot water into the larger pan, reaching about halfway up the sides of the muffin tin. This creates a water bath.
Step 11: Carefully transfer the entire setup to the preheated oven. Bake for 25-30 minutes, or until the tops are golden brown and slightly jiggly in the center.
Step 12: Once baked, turn off the oven and prop the oven door open slightly. Let the cupcakes cool in the oven for at least 30 minutes to prevent sudden temperature changes and shrinking.
Step 13: Remove the muffin tin from the water bath and the oven. Allow the cupcakes to cool completely in the tin on a wire rack before removing.
Step 14: For the best flavor and texture, chill the cupcakes for at least 2-3 hours before serving. Dust with powdered sugar if desired.