Thaw the frozen raspberries, then press them through a mesh sieve into a bowl. You are trying to remove as much of the juice and pulp while leaving as many of the seeds behind as is possible. You should end up with 2/3 cup of juice.

In a small mixing bowl, beat the egg yolks until smooth, then set them aside. Combine the raspberry and lemon juice, sugar, and salt in a medium sized saucepan, then stir and heat over medium-low heat until the sugar has completely dissolved and the mixture is steaming.

Slowly pour the hot juices into the egg yolk while whisking briskly. Be sure not to add it too quickly, or it will cook the egg yolks and create chunks. Pour it all back into the saucepan and cook over low heat, stirring constantly, until the mixture has thickened enough to coat the back of a spoon.

Remove it from the heat, then cut the butter into chunks and stir it in until it has all melted. Pour the curd into a clean bowl and cover it with a sheet of plastic wrap. Refrigerate until completely cool, then store in an airtight container.