1. Pour the condensed milk into a bowl, then steadily add the beer while stirring constantly.
2. Pour the mixture through a sieve into the culinary whipper.
3. Close the culinary whipper, load one N2O gas cartridge and shake for 4 seconds.
4. Refrigerate for 30 minutes before serving.
5. Fill small, clear glasses with chocolate ice cream and top the verrines off with the Guinness espuma.