Place the raspberries in a medium bowl and use a fork to mash them into a puree.
Pour the puree in a strainer over a bowl and press it with the back of a spoon until only seeds remain.
Place the puree in a measuring cup and add the lemon juice. You should have a total of 3/4 cup, so you might have to add more or less of lemon juice. (Idealy 1/2 cup of raspberry juice)
In a medium pan, beat the sugar and egg yolks until well blended. Add the raspberry-lemon juice, lemon zest, butter, and salt. Cook over low, or medium-low heat, stirring constantly until the mixture has thickened.
Pour the curd into a sieve over a bowl.
Allow the mixture to cool at room temperature for one hour, then transfer it into an airtight container.