Butter four ramekins, about 150ml each. Heat oven to 160C/fan 140C/gas 3. Have a roasting tin ready and put the kettle on.
Pour the milk into a pan with the sugar and vanilla. Bring gently to the boil, stirring to dissolve the sugar. Remove from the heat and cool for a few mins.
In a large bowl, beat the eggs until frothy. Slowly whisk in the milk. Set the ramekins in the roasting tin and divide the custard between them. Pour hot water around the ramekins to come halfway up the sides.
Bake for 20 mins until just set, then cool and chill.