Melt the butter in a medium saucepan over medium heat. Remove the pan from the heat and whisk in the sugar, lemon juice, vanilla bean paste, extract, zest, and salt.

Whisk in the yolks until well blended.

Return pan to the stove and over medium-low heat, cook whisking constantly until the mixture is thick enough to coat the back of a wooden spoon or spatula, about 10 minutes without allowing the mixture to boil. This is important. Do not allow the mixture to come to a boil!

Alternatively, you can test for doneness (totally a word) by pouring a few drops of curd onto any surface and running a finger through it. If the curd doesn't come together, it's ready.

Strain the lemon vanilla curd into a clean bowl and cover with plastic wrap, pressing the plastic onto the surface to keep a skin from forming. Let cool to room temperature; then refrigerate until completely chilled.

The curd will keep for up to 3 weeks in the refrigerator.

Adapted from: Fine Cooking