Melt 400g of chopped chocolate and 50g of butter in a bowl over a saucepan of gently-simmering water.
Pour the mixture out onto a baking tray and use a palette knife or spatula to spread it into a smooth, large layer.
Freeze for about 3-4 minutes, until the chocolate is cool and hard. Press your finger into the mixture; it should leave a slight mark, but not an actual depression in the chocolate.
Use a cheese shaver or a sharp-edged metal spatula to begin making shavings from the chocolate. When it begins to curl rather than break, it is ready. Continue making long shavings to curl the chocolate, working quickly while the chocolate is cool. If it becomes too warm, just re-freeze for another minute.