Place the raspberries and sugar into a saucepan over medium heat, and bring to the boil. Simmer for about 5 - 10 minutes, stirring occasionally, until the raspberries have broken down and the mixture has thickened. Push the mixture through a sieve into a small bowl, and discard the seeds and pulp. Allow to cool completely.

Using a handheld or stand mixer, whip the heavy cream to stiff peaks.

In a separate bowl, whisk together the condensed milk, and vanilla. Gently fold in the whipped cream.

Pour half of the mixture into a 9x5-inch loaf pan, and dollop spoonfuls of the raspberry sauce on top. Swirl the sauce through with a toothpick or knife. Pour on the rest of the ice cream mixture, and swirl in more raspberry sauce. Use as much or as little of the sauce as you want, or save some for another batch of ice cream!

Cover tightly, and place in the freezer for at least 6 hours, or overnight before serving.