Step 1

Cook the Raspberries – In a medium saucepan, combine raspberries, sugar, lemon zest, and lemon juice. Stir and cook over medium-high heat until the raspberries break down, about 5 minutes.

Step 2

Thicken the Sauce – Combine cornstarch and water in a small bowl. Add to the raspberry mixture. Stir and cook over medium heat until the sauce has thickened slightly, about 1 minute.

Step 3

Strain – Pour the sauce through a fine mesh strainer into another bowl. Use the back of a spoon to push and separate the sauce from the seeds. Scrape the bottom of the strainer to collect more sauce. It will yield about ¾ cups.

Step 4

Let it Cool – Transfer the raspberry sauce to the refrigerator to cool completely. Meanwhile, make the ice cream base.

Step 5

Flavor the Condensed Milk – In a large bowl, whisk together the sweetened condensed milk with the vanilla and set aside.

Step 6

Whip the Cream – In a stand mixer fitted with the whisk attachment, add the heavy cream. Whisk on medium-high (setting until a fluffy and smooth consistency is reached and stiff peaks form, about 1 1/2 to 2 minutes. It should hold its shape when lifted from the bowl. Do not over-whip!Alternatively, hand whisk by adding the heavy cream to a chilled large metal bowl. Slightly tilt the bowl and vigorously whisk until stiff peaks form, about 3 1/2 to 4 minutes.

Step 7

Make the Ice Cream Base – Gently fold in ⅓ of the whipped cream into the sweetened condensed milk mixture. Add the remaining whipped cream and gently fold in until the mixture is smooth with just a few small lumps left. Do not deflate the mixture by over-mixing!Transfer half of the ice cream base to a 9 by 5-inch loaf pan or container large enough to hold a 1-quart mixture. Dot half of the raspberry sauce onto the ice cream, and use a knife to create swirls. Add the remaining base on the top, dot with the rest of the sauce, and create swirls again.

Step 8

Freeze the Ice Cream – Place a piece of plastic wrap directly on top of the ice cream. Cover with a piece of parchment paper that fits just the surface. Cover the pan tightly with foil or a lid. Freeze until hardened, about 6 hours. Allow to sit at room temperature for a few minutes to soften before scooping.