In a large bowl, mix flour and salt. Add cold butter cubes, working them into the flour until it looks like coarse crumbs. Gradually add ice water, gently mixing until the dough comes together.

Shape the dough into a ball, wrap it in plastic wrap, and chill it for at least 30 minutes.

On a floured surface, roll out the chilled dough to fit a 9-inch pie dish. Place the dough in the dish, trim the edges, and crimp them. Chill while you make the filling.

Preheat the oven to 350F.

In another bowl, mix mashed sweet potatoes, melted butter, sugar, cinnamon, nutmeg, and salt until smooth. Add flax egg, vanilla extract, and milk, mixing until completely smooth.

Pour the sweet potato filling into the prepared crust.

Bake for 45-50 minutes, or until the filling is set and a toothpick inserted in the center comes out clean. Let the pie cool completely on a wire rack.

For the meringue topping, beat aquafaba until frothy. Add salt and cream of tartar, beating until soft peaks form. Gradually add superfine sugar, one tablespoon at a time, until stiff peaks form and the meringue is glossy. Mix in vanilla extract.

Spread meringue over the cooled pie. Use a hand-held torch to caramelize the top.