Step 1

Preheat oven to 375 degrees. Bake sweet potatoes until tender, about 1 hour. Remove from oven and let cool slightly, then discard skins and mash flesh with a fork (you should have 1 1/2 cups).

Step 2

In a food processor, pulse graham crackers until finely ground (you should have about 2 1/4 cups). Add 1/3 cup sugar, 1 stick butter, and 1/4 teaspoon salt and pulse until combined. Firmly press mixture into a 9-by-13-inch baking dish. Bake until crust is dry and set, about 12 minutes. Remove from oven and reduce temperature to 325 degrees.

Step 3

Meanwhile, in clean food processor, puree sweet potatoes, ricotta, yogurt, eggs, 1/4 teaspoon salt, and remaining 3 tablespoons butter and 2/3 cup sugar. Pour into crust and bake until just set in center, 30 to 35 minutes. Let cool on a wire rack, 30 minutes, then refrigerate until cold, 1 hour (or overnight). Cut into squares and top with whipped cream.