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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Line a rimmed baking sheet with 2 overlapping pieces of aluminum foil so the entire sheet and all 4 edges are covered.
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3
Arrange the graham crackers tightly together in a single layer on the baking sheet, using broken-up pieces to fill any empty spaces; set aside.
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4
Place butter, brown sugar, and salt in a small, heavy-bottomed saucepan over medium heat and bring to a full boil, stirring occasionally with a rubber spatula to dissolve the sugar, about 5 to 6 minutes.
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5
Remove from heat, slowly add vanilla (the mixture may bubble up), and stir to combine.
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6
Pour mixture over graham crackers and spread evenly using the rubber spatula.
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7
Evenly sprinkle marshmallows in a single layer over top.
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8
Bake until marshmallows are puffed and just starting to brown, about 8 minutes.
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9
Remove the baking sheet from the oven and place it on a wire rack.
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10
Evenly sprinkle chocolate chips over the marshmallows and let sit until the chips are softened, about 5 minutes.
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11
Lightly drag a toothpick or skewer over the surface and through the softened chocolate chips to create a marbled pattern.
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12
Evenly sprinkle with nuts, if using.
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13
Let cool until warm, about 20 minutes, then refrigerate or freeze until the chocolate has set, about 15 to 30 minutes.
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14
Remove the smore slab from the baking sheet using the foil to lift it in one piece.
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15
Transfer to a cutting board.
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16
Remove and discard the foil.
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17
Using a sharp knife, cut the slab into 2-inch squares.
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18
Store between layers of waxed paper in a container with a tightfitting lid; the bars will keep in the refrigerator for up to 4 days.
