In a small saucepan, combine raspberries, sugar, lemon zest, and lemon juice. Cook over medium-high heat until the raspberries break down, about 8 minutes.
Combine cornstarch and water in a small bowl, then add to the raspberry mixture, stirring until combined. Once the raspberry sauce has thickened slightly, about 2 minutes, then strain the sauce into another bowl to remove the seeds.
Transfer the raspberry sauce to the refrigerator to cool completely. Once cooled begin making the ice cream base.
In a large bowl combine the sweetened condensed milk with the vanilla bean paste, set aside.
Use a mixer, immersion blender or hand whisk on the chilled heavy cream until stiff peaks form. About 2-3 minutes on high in a stand mixer, do not over whip!
Gently fold in about ⅓ of the whipped cream into the sweetened condensed milk. Transfer the rest of the whipped cream into the bowl and gently fold in until the mixture smooths out and becomes soft and silky. Stop folding when you see just a few small lumps left, do not deflate the mixture by over mixing.
Transfer half of the ice cream base to a pan large enough to hold the mixture. Dot half of the raspberry sauce onto the ice cream, and use a knife to create swirls. Add the rest of the cream mixture on top, dot with the rest of the sauce and create swirls again on the top of the ice cream.
Cover with a small piece of plastic wrap. Cover with a small piece a parchment paper that fits just the surface of the ice cream. Cover the pan tightly with more plastic wrap. Freeze until hardend, about 6 hours or overnight.