- Line the bottom of a 20-cm (8-inch) square pan with parchment paper, letting the paper hang over two sides and butter the other two sides.
- In a heavy saucepan, bring all the ingredients except for the chocolate and vanilla to a boil, stirring to dissolve the sugar. Attach a candy thermometer in the centre of the pan and simmer without stirring until the thermometer reads 115 °C (240 °F). Remove from the heat and add the chocolate and vanilla without stirring.
- Place the pan in a cold water bath. Let cool without stirring, until the thermometer reads 43 °C (110 °F) or 20 to 30 minutes.
- Remove the pan from the water. With an electric mixer, beat until the mixture loses its gloss but is still soft, about 2 minutes.
- Pour immediately into the pan and spread with a spatula. Cover with plastic wrap and let cool for 1 hour at room temperature or 30 minutes in the refrigerator. Unmould and cut into 2.5-cm (1-inch) squares.
- Store in an airtight container. The squares of fudge can be made in advance and freeze very well.