Step 1: Preheat your oven to 320°F (160°C) and line a muffin tin with parchment paper liners.
Step 2: In a heat-proof bowl over a pot of simmering water, melt together the cream cheese, butter, and milk until completely smooth.
Step 3: Remove from heat and whisk in the egg yolks one by one until fully incorporated.
Step 4: Sift the cake flour and cornstarch into the cream cheese mixture and whisk until no lumps remain.
Step 5: In a separate clean bowl, beat the egg whites with cream of tartar and lemon juice until frothy. Gradually add the sugar and beat until soft, glossy peaks form.
Step 6: Gently fold the egg whites into the cheese mixture in three stages, using a spatula to keep the batter airy.
Step 7: Divide the batter among the muffin liners. Place the tin in a large pan and fill the pan with hot water (about 1 inch deep).
Step 8: Bake at 320°F for 20 minutes, then lower to 280°F and bake for another 45 minutes until golden and set.
Step 9: Leave in the oven with the door ajar for 15 minutes before removing to cool completely.