Combine raspberries, 1/4 cup granulated sugar, and lemon juice in a medium saucepan. Cook over medium heat, stirring occasionally, until the raspberries break down and the sugar dissolves, about 5-7 minutes.

Press the cooked raspberry mixture through a fine-mesh sieve into a small bowl to remove seeds. Discard the seeds. Let the raspberry puree cool completely, then chill in the refrigerator for at least 30 minutes, or until ready to use.

In a large saucepan, combine heavy cream, whole milk, 3/4 cup granulated sugar, and salt. Heat over medium-low heat, stirring frequently, until the sugar is completely dissolved and the mixture is warm but not boiling. Remove from heat.

Stir in the vanilla extract. Pour the ice cream base into a large mixing bowl. Cover the bowl and refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled. This is crucial for achieving a smooth texture.

While the ice cream base is chilling, finely chop the dark chocolate bar into small, irregular chunks. Place them in a small bowl and set aside in the refrigerator until ready to use.

Once the ice cream base is thoroughly chilled, pour it into the frozen bowl of your ice cream maker. Churn according to the manufacturer's instructions, typically for 20-25 minutes, until the ice cream is thick and resembles soft-serve.

Spoon about one-third of the churned ice cream into a freezer-safe container. Drizzle about one-third of the chilled raspberry swirl over the ice cream, then sprinkle with about one-third of the chocolate chunks.

Repeat the layers two more times, ending with a drizzle of raspberry swirl and a sprinkle of chocolate chunks on top. Use a knife or skewer to gently swirl the raspberry and chocolate through the ice cream, being careful not to overmix.

Cover the container tightly and freeze for at least 4 hours, or until firm enough to scoop.