Nigella's Chocolate Key Lime Pie

Bought a load of limes but cannot for the life of me think why so decided to make a creamy key lime pie. I remembered watching Nigella knocking this up a while ago and the combination of limes and chocolate is sublime.

300g Digestive biscuits
1 Tbsn cocoa powder
50g  Softened butter
50g Dark chocolate chips
1 397g tin condensed milk chilled
 2 Tbsn lime zest
175mls Lime juice
300mls Double cream
1 Square dark chocolate for decoration

1 23cm loose bottomed tart tin ( x 5cm if poss )

Put biscuits, butter and chocolate chips into a food processor and whiz very briefly to big crumbs.
Pour into tin and press into the sides and bottom - not too hard though as it makes the base rock hard
Place into fridge to set
Pour condensed milk into a bowl, add lime juice and whisk
Pour in the cream and whisk until thick, pour into base and chill for at least 4 hours
Just before serving, sprinkle over lime zest and grated chocolate.

Debs