For the base: Place the biscuits in a food processor and pulse until crushed but not too fine. Tip into a bowl, add the melted butter, and stir to combine.
Press the biscuit mixture into the base of a 20-centimeter (8-inch) pie tin, pushing it up the sides of the tin to create a crust. Place in the fridge to set.
Preheat the oven to 150 degrees centigrade/gas mark 2 (300 degrees F).
For the filling, whisk the egg yolks with the condensed milk in a bowl. Add the lime juice and zest and whisk again. Pour into the prepared base.
Bake until just set, 25 to 30 minutes. The filing will have risen and may have started to color around the edges.
Remove from the oven and leave to cool at room temperature, then chill in the fridge for at least 2 hours before serving.
Cut the pie into wedges and serve with whipped cream and a sprinkling of lime zest