Butter, for greasing the pan
1 (14-ounce) can sweetened condensed milk
2 teaspoons ground cinnamon
1 teaspoon pure vanilla extract
1 pound (about 2 cups) bittersweet (60 percent cacao) chocolate chips (recommended: Ghiradelli) see Cook's Note
3 tablespoons unsalted butter, cut into 1/2-inch pieces, at room temperature
Kosher or flake salt, optional
The fudge can also be made using 1 pound (about 2 1/2 cups) of semisweet chocolate chips.