Mix the condensed milk with the cocoa. Sift the cocoa over the condensed milk (this prevents lumps) and stir in the salt until you have a smooth, glossy cream. At first it'll look like it won't come together; keep going a few more seconds and it will.
Whip the cream. With an electric mixer or by hand, to soft peaks. It's better not to over-whip: over-whipped cream is harder to fold in and the ice cream loses its smoothness. The cream should be well chilled; if your kitchen runs warm, chill the bowl too.
Combine the two mixtures. Add the chocolate cream to the whipped cream and fold gently with a spatula until no white streaks remain. Take your time: the more air you keep in, the creamier the result.
Freeze. Transfer the mixture to a container with a lid and freeze for about 4–6 hours, until it reaches the consistency you like. At 4 hours it has a soft-serve texture; at 6, it's scoopable.
