Preheat your oven to 160°C (320°F). Line a 20cm (8-inch) cake pan with parchment paper, ensuring enough overhang for easy removal.
Combine softened cream cheese and melted butter in a bowl, whisking until smooth and creamy with no lumps. Add warm milk gradually and mix until fully incorporated.
Sift the cake flour and cornstarch into the cream cheese mixture, folding gently with a spatula until smooth. Set aside.
Separate the eggs, placing yolks in a small bowl and whites in a clean mixing bowl. Add half of the caster sugar to the yolks and whisk until pale and thick.
Whisk the egg whites with cream of tartar until soft peaks form, then gradually add the remaining sugar while continuing to whisk until stiff, glossy peaks are achieved.
Gently fold a small amount of whipped egg whites into the cream cheese batter to lighten it, then carefully fold in the remaining egg whites until just combined, being gentle to retain air.
Pour the batter into the prepared pan and give it a gentle tap on the counter to release air bubbles. Smooth the top with a spatula.
Bake in the preheated oven for about 50 minutes, or until the top is golden and the cake has a slight jiggle in the center when gently shaken.
Remove the cake from the oven and let it cool in the pan for 10 minutes, then carefully lift it out using the parchment paper and transfer to a wire rack to cool completely. The cake should be soft, airy, and slightly jiggly in the center.
Slice gently to reveal a tender, cloud-like crumb with a shiny, slightly puffy surface. Serve at room temperature and enjoy its melt-in-your-mouth texture.
