Place sweet potatoes on a foil-lined baking sheet and bake until very soft, about 1 hour. When potatoes are cool enough to handle halve and scoop out enough flesh to measure 2 cups, reserving remainder for another use and discarding skin. While potatoes are still hot, add butter and mash until almost smooth; small lumps of potato should remain.
Whisk together eggs, sugar, allspice, ginger, and salt. Stir in evaporated milk, lemon juice, and vanilla. Gradually add egg mixture to sweet potatoes, whisking gently to combine.
Pour the filling into the cooled piecrust. Bake for 20 minutes. Reduce the heat to 325 degrees F and bake until the filling is set around edges but still jiggles slightly when shaken, about 20 more minutes. Remove from the oven and let cool slightly, about 30 minutes.
To make the meringue topping, in a large bowl, beat the egg whites until soft peaks start to form. Slowly add the sugar while beating constantly. Beat until glossy and stiff peaks form, being careful not to over beat the meringue.
Preheat the broiler.
While the pie is still warm spread the meringue evenly over the filling, smoothing out to the pastry edges to seal. Place under the broiler and cook until the meringue is golden brown, about 1 minute.
Remove from the oven and let cool completely on a rack, about 1 hour. Serve.
Gingersnap Crust:Preheat the oven to 375 degrees F.
In a bowl, combine the ginger snap crumbs, butter, and brown sugar and blend well.
Transfer to a 9-inch pie pan, and press into the bottom and up the sides with your fingers. Place another 9-inch pie pan on top of the crust and press firmly to smooth and pack the crust. (Some of the crust may spill over the edges of the pan.)Brush the crust with the egg white.
Bake until golden, 10 to 15 minutes. Remove from the oven and cool. Refrigerate, tightly covered, for one hour before using, or freeze for up to 6 weeks.