Pour the condensed milk into a heavy-based saucepan and sift in the cocoa powder and ¼ of a teaspoon of sea salt.
Place over a low heat and cook for 10 to 15 minutes, stirring, until the mixture is thick, shiny and leaves the sides of the pan when stirred. Do not leave the pan unattended, as the condensed milk can burn easily.
Once the chocolate mixture has thickened, remove from the heat and stir in the butter.
Split the vanilla pod in half lengthways and scrape out the seeds with the back of a small knife. Add the seeds to the pan and stir well. (The scraped-out pod can be used to make vanilla sugar – just put it in your sugar jar and leave it to infuse the sugar).
Let the mixture cool completely, then cover with clingfilm and chill for about 30 minutes, until set.
Put the chocolate sprinkles in a small bowl. If you have one, use a mini ice-cream scoop to make a 20g ball and drop it into the sprinkles (alternatively, use a teaspoon).
Coat it liberally with the sprinkles, roll it with your hands to make a neat ball and place it in a mini cupcake or petit-four case. Continue with the rest of the mixture.
Store in an airtight container in the fridge, but take them out at least half an hour before serving, as they’re best at room temperature.