Place the sugar in a heatproof bowl. Add the lemon zest and use your fingertips to rub the zest into the sugar until it becomes fragrant.
Whisk in the eggs, egg yolks and lemon juice until the mixture is smooth and well combined.
Set the bowl over a saucepan of gently simmering water, making sure the bottom of the bowl does not touch the water. Cook, whisking constantly, until the curd thickens and reaches 75°C. This will take approximately 10 minutes.
Remove the bowl from the heat and whisk in the butter, one cube at a time, allowing each piece to melt completely before adding the next.
Transfer the curd to a blender and blend on high for 1 minute. Scrape down the sides of the blender and repeat until you have blended for a total of 3 minutes. The curd should be beautifully silky smooth.
Pour the curd into a clean container and press a piece of cling wrap directly onto the surface of the curd to prevent a skin from forming.
Refrigerate until completely chilled or until ready to use. The whipped lemon curd will keep in the refrigerator for up to 1 week.
