To make the base, put the biscuits into a strong plastic bag and bash with a rolling pin until you get crumbs.
Melt the butter and mix well with the biscuits.
Preheat the oven to 160 degrees C (320 degrees Fahrenheit).
Use either a 20cm loose base tart tin or 2 smaller tins and spread the biscuit mixture over the bottom of the tin, making sure you cover up the sides as well. Press firmly with the tip of your fingers or a spoon.
Bake in the oven for 10 minutes, then cool completely.
To make the filling, add the egg yolks to a bowl and beat with an electric mixer until pale yellow and double in volume.
Add the condensed milk and beat again for 1 minute, then add the lime zest and juice and beat again until everything is fully incorporated.
Pour the filling into the biscuit base, then put the tin back in the oven for 12-15 minutes until the filling is set.
Remove from the oven and leave to cool.
Beat the whipped cream with icing sugar until you get stiff peaks.
Decorate the pie with whipped cream using a piping bag fitted with a nozzle of your choice.
Grate some more lime zest over the pie if you like.