Preheat oven to 180C. Prepare and chill coffee if not already done. Grease and baseline a 21 x 21 x 4cm baking tin. Put butter and sugar in a bowl and whisk with electric beaters until pale and fluffy. Whisk in milk.
Add flour and salt and mix with a wooden spoon until mixture forms a dough. Spoon into the tin and spread out smoothly using a blunt knife. Bake for 20 to 25 minutes until lightly browned around the edges. Increase oven temperature to 190C. To make cheesecake topping, combine the cream cheese, egg yolks and caster sugar using an electric whisk. Gradually whisk in coffee until the mixture is smooth.
Add the flour and chocolate and mix in. In a separate bowl, whisk egg whites until firm peaks form then gently fold into the mixture. Pour over the biscuit base. Bake for 35 minutes. Cool in the tin. Makes about 15 pieces.