Puree the berries in a food processor or blender. Run the puree through a sieve or the fruit strainer attachment of your mixer to remove the seeds. You should end up with about 1 ¼ cups of seedless puree. Measure out ½ cup of the puree for the curd and reserve the rest for another use. Stir the lemon juice into the ½ cup of puree.
If you're using the gelatin, bloom in 2 tablespoons cold water in a medium size bowl. Set aside.
Thoroughly whisk together the sugar and eggs in a medium saucepan. Add the 1/2 cup raspberry puree and salt to the pan.
Place the butter on top of the bloomed gelatin or in a clean bowl if you're not using the gelatin. Put a fine mesh sieve over the bowl and keep it near the stove.
Heat the egg/raspberry mixture over medium-low heat, stirring constantly. You must be sure to keep stirring across the entire bottom and corners of the pot so the custard does not have a chance to stick. The mixture will start out opaque with a fine white foam over the surface. As it heats up the curd will thicken and the foam will disappear.
Cook until the curd is just beginning to boil. Don't allow it to come to a full boil, just 1 or 2 bubbles and you're ready.
Immediately remove from the heat and pour through the sieve over the butter and (optional) gelatin.
Stir until the butter and gelatin are melted and thoroughly incorporated. Cover the bowl with plastic wrap and cool a bit before putting in the refrigerator.
Use as needed in your favorite recipes or as a spread.