Preheat the oven to 180C/350F/Gas 4.
For the pastry base, finely crush the biscuits by placing into a plastic bag and bashing with a rolling pin (alternatively, pulse to crumbs using a food processor). Transfer to a mixing bowl.
Melt the butter in a saucepan, then pour over the biscuit crumbs. Add the sugar, then mix well to combine. Evenly spread the biscuit mixture in a 23cm/9in pie case, making sure to cover the sides of the tin as well. Level off any excess biscuit mixture.
For the filling, whisk the egg yolks in a bowl until pale and fluffy. In a separate bowl, whisk together the lime zest, lime juice and condensed milk until smooth and creamy, then mix into the egg yolk mixture. Pour the mixture into the pie case.
In a clean bowl, whisk the egg whites until soft peaks form when the whisk is removed from the bowl. Gradually fold in the sugar, vanilla extract, salt and cream of tartar, then whisk again until stiff peaks form. Spoon the meringue mixture over the filling in the pie case, using a fork to form the meringue into peaks.
Bake the pie in the oven for 15-20 minutes, or until the meringue is golden-brown. Remove from the oven and allow to cool. Place into the fridge to chill completely before serving.
To serve, cut slices of the key lime pie and serve with a scattering of blueberries and a few spoonfuls of whipped cream.