To a medium (2-quart) saucepan, add the granulated sugar, confectioners sugar and lemon zest whisking to combine.
Whisk in the egg yolks until a thick paste forms and then slowly pour in the lemon juice, continuing to whisk all the while. Place the saucepan over low heat (see note below recipe regarding temperature) and whisk for 10 to 15 minutes or until the curd starts to thicken and coats the back of a spoon. Resist the temptation of turning up the heat as it will yield a curd that isn't smooth and fabulous.
Once it has started thickening, take it off of the heat and whisk in the butter.
Ideally, pour through a fine-mesh sieve (if you have one) and then into a lidded airtight container. Place a layer of plastic wrap directly over the top so that it doesn't form a 'skin' (once thoroughly chilled you can remove and discard the covering). Chill in the fridge for at least 3 to 4 hours before using.