Preheat the oven to 180°C/160°C fan/350°F/gas 4. Crush the digestive biscuits and mix with the melted butter, then press into a 23cm /9 inch loose-based flan tin or springform cake tin. Bake in the oven for 8-10 minutes, then leave to cool while you make the filling.
In a large bowl, add the egg yolks, 2 pots of lemon yogurt, the tin of condensed milk, juice of 2 limes and zest of one. Whisk together until completely blended. Pour the mix onto the biscuit base, then bake in the oven for 15-18 minutes until set. Remove and chill in the fridge for at least one hour.
To make the topping, whip the cream along with the remaining pot of lemon yogurt until it forms firm peaks. Generously cover the pie and sprinkle with a little lime zest.