Cut each toffee into 4 pieces and put in a bowl with the salt flakes. Roll the toffees around so some salt flakes stick to each.
Put the chocolate and cream in a bowl and melt in 20-second blasts in the microwave, or set over a pan of simmering water (make sure the base doesn't touch the water). Remove and pour into a container that will give the mixture a depth of at least 2cm (¾in). Leave to cool completely. Once cool, put in the fridge to chill for 2-3 hours, until the mixture is firm enough to shape.
Using a melon baller or teaspoon, scoop out the balls. Push a piece of toffee into each ball and smooth over the indent. Roll each truffle in cocoa powder (if using your hands dust them with cocoa powder first).