You'll need a large, heatproof glass bowl and a pot in which the bowl can rest without touching the bottom. Fill the pot with one inch of water. Bring to a simmer over medium heat.

In a food processor or mortar and pestle, combine the sugar and lemon zest and pulse or mash until the sugar is yellow and the delicious smell of lemon sugar almost knocks you out, about a minute in the food processor or a few minutes in the mortar.

Combine the lemon sugar, eggs, and egg yolks in the heatproof bowl. Whisk together for one minute to distribute the sugar.

Place the bowl over the pot with simmering water and whisk constantly for about 30 seconds, until the sugar is dissolved.

Add the lemon juice and lime juice and cook, whisking frequently, for about 10 minutes, until the curd reads 160° F on an instant read thermometer and has the consistency of sour cream. Remove the bowl from the heat.

Whisk in the pieces of cold butter one by one until they are completely incorporated.

Strain the curd through a fine-mesh sieve into a clean bowl.

Curd is ready to use in recipes or transfer to the refrigerator or freezer.