Using a either a blender or stick blender, puree the blackberries until smooth. Alternatively, mash blackberries using a potato masher. Pour blackberries into a fine mesh sieve set over a pot. Using a spatula, press pureed blackberries through the sieve, discarding seeds and any remaining pulp.

Whisk in the sugar, salt egg, egg yolks and lemon juice. Bring to a simmer over low heat, whisking occasionally. Allow to thicken until the mixture coats the back of a wooden spoon.

Remove from heat and mix in butter. Mixture will continue to thicken as it cools. Once cooled, store in the refrigerator in an airtight container.