Preheat your oven to 160°C (325°F). Line a 12-cup cupcake pan with paper liners. Place the cupcake pan inside a larger roasting pan. Bring a kettle of water to a boil for the water bath.

In a large heatproof bowl set over a saucepan of simmering water (double boiler, ensuring the bowl does not touch the water), combine cream cheese, butter, milk, and 40g sugar. Stir continuously with a whisk until the mixture is smooth, melted, and lump-free. Remove from heat.

Whisk in the egg yolks one at a time into the cream cheese mixture until well combined. Add the lemon juice and vanilla extract, whisking until fully incorporated.

Sift the cake flour and cornstarch directly into the cream cheese mixture. Whisk until the batter is smooth and no lumps remain. Set aside to cool slightly while you prepare the meringue.

In a clean, dry medium mixing bowl (preferably a stand mixer bowl), add the egg whites and cream of tartar (if using). Beat on medium speed until foamy.

Gradually add the 70g sugar, a tablespoon at a time, while continuing to beat on medium-high speed. Continue beating until stiff, glossy peaks form (when you lift the whisk, the peaks should hold their shape without drooping, but still be slightly pliable).

Gently fold one-third of the meringue into the cream cheese batter using a rubber spatula to lighten it. Then, carefully add the remaining meringue in two additions, folding gently until just combined. Be careful not to deflate the meringue.

Divide the batter evenly among the 12 lined cupcake cups, filling each about three-quarters full.

Pour hot water from the kettle into the larger roasting pan, surrounding the cupcake pan, until it reaches about halfway up the sides of the cupcake pan.

Carefully transfer the roasting pan to the preheated oven. Bake for 20 minutes at 160°C (325°F), then reduce the oven temperature to 135°C (275°F) and bake for another 35-40 minutes, or until the tops are golden brown and a skewer inserted into the center comes out clean (or with moist crumbs, not wet batter). The cupcakes will puff up significantly.

Turn off the oven, crack open the oven door slightly, and let the cupcakes cool in the water bath inside the oven for 30 minutes. This helps prevent cracking and shrinking.

Carefully remove the cupcake pan from the water bath and place it on a wire rack. Let the cupcakes cool completely in the pan before removing them from the liners.