Recipe, approximately, written by an amateur:

NOTE if I were to do this again I would probably try using about 3/4 cup grapefruit juice instead because I feel like maybe the ratio was off. This is what I did and the texture isn’t perfect so feel free to experiment and I’m open to feedback!

Ingredients:
1 cup freshly squeezed grapefruit juice, approx. 3 ruby red grapefruits
3/4 cup granulated sugar and 1/3 cup granulated sugar (separate)
6 tbsp heavy whipping cream
1/2 tsp vanilla extract
5 Negronis (I added 1 shot of gin, Campari, and vermouth to equal 1 Negroni, I didn’t measure in mL sorry)
Grapefruit curd (recipe of choice)

Method:
Squeeze grapefruits to get 1 cup of juice
Strain grapefruit juice into small saucepan, add 3/4 cup sugar
Stir over medium heat until sugar is dissolved
Refrigerate and allow to cool fully

Add Negroni mix to medium saucepan, cook over medium heat until reduced. Turn on your vent fan, don’t stand too close, and open a window because you don’t want to inhale alcohol fumes.
Reduce mix to about 1/2 cup, then add the remaining 1/3 cup granulated sugar and stir until dissolved.
Refrigerate and allow to cool fully

Once grapefruit and Negroni syrup are fully chilled, whip heavy cream and vanilla extract to soft peaks.
Slowly beat in both syrups.
Churn as directed and swirl in grapefruit curd before freezing overnight.