In medium bowl, massage lemon zest into sugar and let rest.
In sauce pot, heat lemon juice just until starting to bubble.
Once lemon juice is hot, you can add your yolks, starch and salt to sugar mixture and whisk until fully incorporated. (if you add yolks too early, the sugar can burn the causing small flecks/chunks in your curd).
Steadily whisk the hot lemon juice into your egg yolk mixture in a slow stream until fully incorporated.
Pour the lemon/yolk mixture back into the saucepan and whisk constantly on medium heat until mixture is thickened and coats the back of a spoon. Should take 3-5 mins.
Once thick, remove from heat and add vanilla.
Strain mixture through a sieve and place bowl over ice bath or gel mat to cool. Once mix is room temp, cover with cling film and refrigerate until completely cooled.
