Heat a medium saucepan over medium-high heat. Add the blackberries and sugar and bring to a low simmer. Use a wooden spoon to crush the blackberries as they start to soften and release their juices.** Simmer for approximately 5 minutes, stirring frequently to prevent the sugars from burning.
To the pot stir in the lemon juice, heavy cream, milk and salt. Bring to a gentle simmer (not a boil!)
Have the egg yolks ready and waiting in a medium to large mixing bowl. Carefully whisk 1/3 cup of the warm cream and blackberry mixture in with the egg yolks, whisking constantly to prevent eggs from cooking. Repeat this process, 1/3 cup at a time, until approximately 2 cups of the cream and blackberry mixture have been mixed with the egg yolks. Do not rush this step :) After two cups have been added, whisk in remaining cream and blackberry mixture and stir well.
Cover and chill the mixture in the refrigerator for 4 hours to overnight.
Once cool, pour the mixture into desired dish- brownie pan, bread pan, etc. and add the chocolate chunks.
Transfer to the freezer and, every 30 minutes or so, beat with an electric mixer until fluffy. Repeat this step 3-4 times (approximately 2 hours later), and sprinkle with remaining chocolate chunks and blackberries, if desired.
Cover and freeze until solid.