1. Prepare the Crust:

1. Preheat oven to 200°F (95°C). Line a large baking sheet with parchment paper.
2. In a clean, dry bowl, beat egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form.
3. Gradually add sugar, 1 tablespoon at a time, beating on high until stiff, glossy peaks form and sugar is dissolved.
4. In a small bowl, mix mashed purple sweet potato, vanilla extract, and salt until smooth. Gently fold the sweet potato mixture into the meringue in a few strokes, leaving visible swirls.
5. Spread the meringue mixture onto the prepared baking sheet into a thin, irregular layer about 1/4-inch thick. Bake for 2 hours, then turn off the oven and let meringue cool completely inside for 1-2 hours until crisp.
6. Break into bark-like pieces and store in an airtight container.