Place digestive biscuits, salt and sugar in a large bowl, combine well. Melt butter in the microwave and add to the bowl with biscuits. Mix till crumbs are coated. Transfer the crust into 9-inch cake or pie dish. Bake for 5 minutes. Set it aside while you are preparing filling.
Zest limes, you should get 2 tbsp. Place egg yolks and lime zest in a bowl of your electrical mixer, beat 5-6 minutes until smooth and pale.
Add 1 can of Sweetened condensed milk. Again beat 4-5 minutes until thickened.
Squeeze limes. You will get 160-170 ml. Whisk in yolk mixture.
Pour into crust, bake another 10 minutes. It should not be browned in the end. Let Key lime Pie cool completely before adding the topping.
In your electrical mixture beat whipping cream and sugar till soft peaks. Pipe pie the way you want.