1. Preheat oven to 175C (350F, Gas mark 4). Line a 7 inch cake tin with greaseproof paper.
2. Soften the cream cheese with milk in a bowl. Add half of the caster sugar, egg yolks, cornflour, lemon juice and cream of tartar and combine together
3. Place the egg whites in a large bowl, whisking them until they form stiff peaks and then keep whisking, adding the remaining sugar in 2-3 batches until the mixture stands in stiff peaks.
4. Fold the hald of the egg white mixture into the cream cheese mixture as gently as possible, then fold in the remaining egg white mixture gently but thoroughly.
5. Transfer the mixture to the prepared tin and level the surface. Put the tin into a larger roasting tin and create a bain-marie by pouring boiling water in the roasting tin. Bake on the lower shelf in the oven for 35 to 40 min. Leave in the tin until cool enough to handle.