Step 1

Fill a medium saucepan one-third with water; bring to the boil. Place cream and milk in a medium heatproof bowl; place the bowl over the pan. Heat cream mixture until it reaches 85°C/185°F on a cooking thermometer. Remove thermometer, then the bowl from the pan; cool for 10 minutes.

Step 2

Stir tartaric acid and the water together in a small cup. Gently stir mixture into cream mixture; stand for 20 minutes. (The mixture will form fine loose curds and look curdled.)

Step 3

Line a medium sieve with two layers of muslin; place over a medium bowl. Gently ladle cream mixture into the bowl; cover with plastic wrap. Refrigerate for 8 hours or overnight until mixture has drained and thickened. Transfer mascarpone to a sterilised airtight container; refrigerate.