In a medium saucepan over medium high heat, combine butter and cream. Bring to the boil and remove from heat. Stir in 100g chopped chocolate, Grand Marnier and orange zest; continue stirring until smooth. Pour truffle mixture into a shallow baking dish. Chill until firm, about 2 hours.
Line a baking tray with greaseproof paper. Shape chilled truffle mixture by rounded teaspoons into small balls (a melon baller also works well for this part). Place on prepared baking tray. Chill until firm, about 30 minutes.
In the top of a double boiler over lightly simmering water, melt remaining 100g chocolate with the oil, stirring until smooth. Cool to lukewarm.
Drop truffles, one at a time, into melted chocolate mixture. Using 2 forks, lift truffles out of the chocolate, allowing any excess chocolate to drip back into the pan before transferring back onto baking tray. Chill until set.