1) Cream the sweet potatoes with an electric mixer. This will produce a smooth, creamy pie with no lumps.
2) Preheat the oven to 180 degrees C/Gas 4.
3) In a large bowl, combine the potatoes, 200g of the sugar, butter, eggs, vanilla, salt, and spices. Mix thoroughly. Add the milk, stirring until well combined.
4) Pour the mixture into the pie crust and bake for 40 to 50 minutes, or until a knife inserted into the centre comes our clean. Place the pie on a rack and cool to room temperature before covering with meringue.
For the meringue:
1) Using an electric mixer, beat the egg whites until soft peaks form; beat in the remaining 50g sugar, 1 tablespoon at a time. Continue beating until the sugar dissolves and the mixture is glossy and stiff, but not dry.
2) With a rubber spatula, spoon the meringue mixture on the pie, forming peaks. Make sure the meringue touches the crust all around.
3) Sprinkle with a pinch of sugar. Bake for 5 to 10 minutes, or until delicately browned. Cool and serve.