Mix the vanilla extract with the condensed milk.
Chop the dark chocolate with a knife until very very small. Don't do this using a food processor or the friction will melt the chocolate. Place in the freezer while we whip the cream.
Whip the cream up until fluffy and you can form medium peaks.
Mix ⅓ of the whipped cream with the condensed milk. Then add the rest and fold it in using a spatula.
Add the chocolate and mix it in.
Pour the mix into a freezer-safe container for at least 8 hours.