Pour the sugar and lemon zest into a non-reactive saucepan. Using clean hands, rub the sugar and lemon zest together until it's fragrant and the lemon zest has released its oils, about 1-2 minutes.

Add the egg yolks to the lemon sugar. Using a rubber whisk, whisk the egg yolks and sugar together until the mixture is thickened and pale yellow, about 2-3 minutes. Whisk in the lemon juice until the mixture is smooth. Then, stir in the salt.

Cook the lemon curd over medium-low heat, whisking constantly, until the mixture thickens, reaches 170°F (77°C), and coats the back of a wooden spoon. Don't let the curd come to a boil!

Remove the pot from the heat and add the cold cubes of butter. Whisk until the butter is completely melted. Then, stir in the vanilla bean paste.

Strain the lemon curd through a fine-mesh sieve to remove any lumps or lemon zest. Let the curd cool for a few minutes. Then, pour the lemon curd into a jar and press a small piece of plastic wrap against the top to keep a film from forming. Seal the jar and refrigerate the curd for up to 1 week.