Bash the biscuits to crumbs or blend them in a food processor.
Mix in the melted butter and press into the base and up the sides of a 22cm loose-based tart tin.
Bake in the oven for 10 minutes. Remove and cool.
Crack 3 egg yolks in a large bowl and whisk for a minute with electric beaters. Pour in the condensed milk and whisk for a further 3 minutes, then add the finely grated zest and juice of 4 limes and whisk again for 3 minutes.
Pour the filling into the cooled base then put it back in the oven for 15 minutes.
Cool then chill for at least 3 hours to set.
When you are ready to serve, whip together the double cream and icing sugar. Dollop the cream on top and finish with extra lime zest.