Set to 325°F (163°C) and line a muffin tin with cupcake liners.
Melt the base ingredients: Over a double boiler, whisk cream cheese, butter, and milk until smooth. Remove from heat and let cool slightly.
Mix the batter: Whisk in egg yolks, then sift in flour and cornstarch. Stir until smooth.
Whip the egg whites: Beat egg whites and cream of tartar until foamy. Gradually add 1/4 cup sugar and beat until soft peaks form.
Fold and combine: Gently fold the meringue into the batter in three additions, keeping it airy.
Fill the liners: Pour batter into cupcake liners, filling each 3/4 full.
Prepare the water bath: Place the muffin tin in a larger baking pan. Pour hot water into the outer pan until halfway up the muffin tin.
Bake for 20-25 minutes until lightly golden and set.
Turn off the oven, crack the door, and let cupcakes sit for 10 minutes. Then, cool completely before dusting with powdered sugar.