Cook the sweet potatoes in a large pot of boiling water until fork-tender, about 25-30 minutes. Drain and let cool completely.
Preheat the oven to 365˚F (185˚C).
Peel the sweet potatoes, then transfer to a large bowl and mash thoroughly with an electric hand mixer or fork.
Add the melted butter, sugar, nutmeg, cinnamon, and salt. Beat to combine, then scrape down the sides of the bowl with a rubber spatula. Add the eggs, 1 at a time, beating to incorporate each addition. Add the self-rising flour, buttermilk, vanilla, and lemon extract, and beat to combine.
Divide the sweet potato filling between the frozen pie crusts and smooth the tops.
Bake for about 1 hour, until pies are set in the center. Cover the crusts with foil if it is getting too dark before the pies finish baking.