Preheat oven to 160C (140C fan). Cut a circle of baking (non-stick greaseproof) paper to cover the bottom of a 7-8 inch cake tin. Then, cut some strips to line the sides of your tin – stick them well with some butter or oil.
Crush your biscuits, either by bashing your packet with a rolling pin or using a blender. Stir the crumbs with your melted butter, then press firmly into a flat disc to coat the bottom of your cake tin. Place in the oven for 10 minutes to set it.
Zest and juice the limes while the base is baking. Once done, remove it to cool slightly while you make the rest of the filling. Simply whisk together the yolks, zest, juice and condensed milk until smooth. Pour this on to your base and place back in the oven for 15-20 minutes.
Once the filling is set, remove the pie from the oven and leave it to cool completely before chilling in the fridge.
To serve, whip your cream until thick – be careful because cream overwhips easily. Put a dollop on top, then sprinkle with a little lime zest.