Whip cold heavy cream to medium-stiff peaks (3–4 minutes).
Whisk sweetened condensed milk, vanilla, and salt in a separate bowl, then gently fold into whipped cream.
Pour into a 9x5 inch loaf pan and smooth the top.
Cover with plastic wrap pressed onto the surface and freeze for at least 4 hours.
Scoop and serve with mix-ins folded in for that classic McFlurry texture.
Notes
Use heavy cream with at least 35% fat for the best texture. Don't freeze longer than 6 hours without checking — let sit at room temp before scooping if too hard.