Line a baking dish with paper. Bring kettle to the boil.
Break up the chocolate and place in a dry, heatproof bowl with butter, peanut butter and vanilla. Pour kettle water in a small saucepan until 3 cm deep and bring to a gentle simmer. Place the heatproof bowl over the saucepan. Use a dry spoon to stir and slowly melt the chocolate and butter, stirring to combine with the peanut butter.
Pour evenly into a baking dish and sprinkle with sea salt (if using). Place in the freezer for a minimum of 30 minutes until set.
Remove from tray and cut into 16 pieces. Store in an airtight container in the freezer.