Prick the sweet potatoes all over and roast on a foil-lined sheet at 400F for 50 to 60 minutes, until the skin puckers and syrup leaks onto the pan.

Cool, peel, and blend or mash until completely smooth. Chill the puree until cold. If the puree is fibrous, push it through a mesh sieve.

Whisk the chilled sweet potato puree, sweetened condensed milk, brown sugar, vanilla, cinnamon, nutmeg, and salt together in a large bowl. Taste and adjust the spice now.

In a separate cold bowl, beat the heavy cream on medium-high for 2 to 3 minutes, until it holds soft peaks. Do not whip to stiff peaks.

Stir about one third of the whipped cream into the sweet potato base to loosen it, then fold in the remaining cream in two additions. Stop as soon as the color is even.

For the optional swirl, stir the crushed graham crackers into the melted butter, brown sugar, and cinnamon until evenly coated.

Pour half the base into a 9x5-inch metal loaf pan, spoon over half the graham mixture, then repeat with the remaining base and graham mixture. Drag a butter knife through the pan in an S shape two or three times only.

Press plastic wrap directly onto the surface and freeze at least 6 hours or overnight. Let the pan sit at room temperature for 10 minutes before scooping.