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1
Preheat the oven to 375u00b0F.
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2
Rub the sweet potatoes with the vegetable oil and place on a baking sheet lined with parchment paper. Bake until fork-tender, 1 to 1 1/2 hours. Remove from the oven and let cool, then peel and mash in a large bowl.
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3
Add to the mashed potatoes the brown sugar, cinnamon, nutmeg, salt, vanilla, eggs, and 2 1/2 cups of the milk and mix well. Set aside.
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4
Lower the oven temperature to 350 degrees F.
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5
Combine the graham cracker crumbs and pecan pieces in a food processor and process for about 1 minute, to make a fine meal. With the motor running, gradually pour the melted butter through the feed tube and process for 1 minute. Transfer the crumb mixture to a 13 X 18-inch baking pan and, using your fingers, firmly press the crumbs evenly onto the bottom and up the sides of the pan.
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6
Pour the sweet potato mixture into the crust and spread evenly with a rubber spatula.
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7
Put the chocolate morsels and the remaining 1/2 cup milk in a medium-size saucepan over medium heat and stir constantly until the chocolate melts and the mixture is smooth. Spoon the chocolate in three rows, about 2 inches apart, lengthwise over the sweet potato mixture. Then, with a knife, make a zigzag pattern to marbleize the chocolate and the sweet potato mixture.
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8
Bake until the edges are browned and the filling is set, 20 to 25 minutes. Remove from the oven and let cool completely. To serve, cut into bars.
