For the terrine, lightly grease a 1.5kg/3lb 4oz loaf tin with a little oil and line with cling film, pressing into the corners.
Place the chocolate into a heatproof bowl set over a pan of simmering water to melt (make sure the bottom of the bowl doesn't touch the water). Once melted, remove the bowl from the pan and stir in half of the double cream. Set aside.
Place a clean heatproof bowl over a pan of simmering water (again making sure the bottom of the bowl doesn't touch the water), add the egg yolks and sugar and whisk for 3-4 minutes, or until the mixture is pale and thickened.
Whisk the egg and sugar mixture into the chocolate until well combined, then whisk in the cold water.
Whisk the remaining double cream in a bowl until soft peaks form when the whisk is removed. Gently fold the whipped cream into the chocolate mixture until just combined.
Dip the boudoir biscuits or sponge fingers into a bowl of the chilled coffee, then arrange the soaked biscuits vertically along the sides of the prepared loaf tin. Make sure there are no gaps between each biscuit.
Spoon the chocolate mixture into the centre, then tap the tin lightly on the work surface to even out the mixture, ensuring the mixture goes into the corners. Cover with cling film and chill in the fridge for at least two hours, or until set.
For the coffee crème anglaise, place the double cream, milk and roasted coffee beans into a pan and bring to a simmer.
Whisk the egg yolks and sugar together in a bowl, then slowly pour in the warm cream mixture, whisking continuously until well combined. Pour the mixture back into a clean pan and cook over a low heat for 4-5 minutes, or until the mixture coats the back of a wooden spoon.
Pass the mixture through a fine sieve into a bowl and allow to cool slightly before chilling in the fridge for at least 30 minutes.
To serve, remove the terrine from the fridge 30 minutes before serving to allow it to soften slightly. Turn out from the tin onto a plate and remove the cling film. Using a hot knife, thickly slice the terrine and place a slice onto each serving plate. Spoon the chilled crème anglaise around the plate and decorate the plate with a few chocolate-coated coffee beans.