Step 1: Prepare Oven & Pan
Line a 12-hole muffin tin with cupcake liners.
Preheat oven to 160°C (320°F).
Place a tray of hot water on the lowest rack for steam.
Step 2: Make the Cream Cheese Mixture
Over a simmering water bath, melt cream cheese, butter, and milk until smooth.
Remove from heat, whisk in yolks and vanilla.
Sift in flour + cornstarch, whisk until just combined.
Step 3: Whip Egg Whites
Beat egg whites with cream of tartar until foamy.
Gradually add sugar (about 70 g) and beat to stiff, glossy peaks.
Step 4: Fold Gently
Fold ⅓ of the whites into the cream cheese mixture to lighten.
Add remaining whites in two more additions, folding carefully.
Step 5: Bake
Divide batter into liners, almost to the top.
Place muffin tin on middle rack above the water tray.
Bake 25–30 minutes until golden and slightly jiggly.
Step 6: Cool Gradually
Turn off oven, leave door ajar 10 minutes.
Remove and cool fully before serving.