Prep the pan and oven: Line a 12-cup muffin tin with paper liners. Heat the oven to 300°F (150°C). Place a rimmed baking sheet or roasting pan on a lower rack for the water bath.

Warm and smooth the base: In a heatproof bowl set over a pot of gently simmering water, whisk together cream cheese, butter, and milk until silky and lump-free. Remove from heat and cool 3–4 minutes.

Whisk in the yolks: Add egg yolks, vanilla, and a pinch of salt. Whisk until blended and glossy.

Sift the dry ingredients: Sift cake flour and cornstarch over the bowl. Whisk just until smooth. The batter should be thick but flowy.

Make the meringue: In a clean bowl, beat egg whites with cream of tartar until foamy. Gradually rain in half of the sugar, then the rest, beating to soft, shiny peaks. The tip should curl, not stand stiff.

Fold gently: Add one-third of the meringue to the batter and whisk lightly to loosen. Fold in the rest with a spatula using broad, gentle strokes. Do not deflate.

Fill the liners: Divide batter evenly among liners, about 3/4 full. Tap the pan lightly to release large bubbles.

Make the water bath: Pour hot tap water into the preheated baking sheet to create a shallow bath. Set the muffin tin on the middle rack above it. This keeps the oven humid and heat even.

Bake low and slow: Bake 20–24 minutes until the tops look set, lightly golden at the edges, and spring back when touched. They may rise and dome slightly.

Rest in the oven: Turn off the oven, crack the door, and let cupcakes sit 10 minutes. This gentle cool prevents collapse.

Cool fully: Transfer to a rack. They’ll settle into a cute, slightly wavy top. Dust with powdered sugar or add berries once cool.