Place the biscuits in a food processor or blender and whizz until small crumbs form. Alternatively, place the biscuits in a plastic food bag, seal and crush with a rolling pin. Mix together the biscuit crumbs and melted butter, then press into the base and sides of a 22 cm (8½ inch) loose-bottomed tart tin. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 10 minutes until set and lightly browned. Leave to cool.
Mix together the condensed milk, egg yolks, lime rind and juice, tequila and liqueur in a large bowl until well combined. Whisk the egg whites in a separate clean bowl until stiff peaks form, then gently stir into the lime mixture in 3 batches.
Pour the mixture into the prepared base, then return to the oven and bake for 15–20 minutes until the centre is just set. Leave to cool, then chill for at least 2 hours or preferably overnight.
When ready to serve, whip the cream until soft peaks form. Remove the pie from the tin and place on a serving plate. Swirl over the cream, then arrange lime slices on top to decorate.