Bring a pan half-full of water to the boil and then lower the heat to the absolute minimum. Put the egg yolks and sugar in a wide glass or stainless steel bowl and whisk until they are thick and creamy. Sit the bowl on top of the pan of water, add the butter and let it melt. Whisk until it starts to get thicker and creamier, then add the lemon zest and juice and carry on whisking until it thickens. It doesn’t have to be cooked out like a proper lemon curd, but just until it has thickened and the egg is cooked through from the warmth of the simmering water.
Meanwhile, warm up the milk
Cover the bowl and put in the freezer. After an hour give the mixture an energetic whisk with a hand whisk or electric mixer. Put it back in the freezer and whisk again after another couple of hours. When the ice cream is nearly firm, give one last whisk and put it back in the freezer to set.
Alternatively, pour the mixture into your ice cream machine and churn, following the manufacturer’s instructions.