Crush the biscuits into fine crumbs using a rolling pin or food processor.
Mix crumbs with sugar, then pour in melted butter and combine well.
Press mixture into a 23cm springform tin, forming a base and sides. Chill for 20 minutes.
Preheat oven to 170°C (325°F).
In a bowl, whisk condensed milk, lime juice, zest, elderflower cordial, egg yolks, and half the cream until thickened.
Pour filling into crust and gently level the surface.
Bake the Key Lime and Elderflower Pie for 15–20 minutes until slightly set with a gentle wobble.
Cool for 30 minutes, then refrigerate until fully chilled.
Whip remaining cream with icing sugar until soft peaks form.
Spoon whipped cream over pie and garnish with mint before serving.