Line a 6" Charlotte tin with the boudoir biscuits, having first dipped them in melted butter to ensure that they stick to the tin.
Combine the sugar, orange juice and rind in a saucepan. Bring to the boil for 3-4 minutes.
Place the yolks in an electric beater. Pour on the orange syrup, beating fast. Continue for 10 minutes. Add the Grand Marnier and beat. Lastly fold in the whip