Preheat oven to 166 °C (165 °C). Prepare a water bath by placing a deep baking pan filled with hot water on the lower rack. Line an 8-inch round cake pan with parchment paper on the bottom and sides.

In a saucepan over low heat, melt together cream cheese, butter, and 60 ml sugar until smooth. Remove from heat and let cool slightly.

Whisk egg yolks into the warm cream cheese mixture one at a time until fully incorporated. Sift in cornstarch and flour, then stir until no lumps remain. Add milk and vanilla (if using); mix until batter is silky and pourable.

In a clean, dry bowl, beat egg whites with cream of tartar until foamy. Gradually add remaining 60 ml sugar while increasing mixer speed from medium to high. Beat until stiff, glossy peaks form.

Gently fold 1/3 of the meringue into the yolk batter to lighten it. Then, pour this mixture back into the remaining meringue and fold carefully until just combined—no white streaks, but don’t overwork it.

Pour batter into prepared pan and tap gently to release air bubbles. Place in the oven (on the rack above the water bath) and bake at 166 °C for 25 minutes.

After 25 minutes, briefly crack the oven door open for 10 seconds to release steam. Reduce temperature to 121 °C (120 °C) and bake for another 45 minutes.

Turn off the oven and let the cake rest inside for 10 minutes. Then remove, invert onto a plate lined with parchment, peel off the old paper, and re-invert onto your serving plate.