Line 2 baking sheets with waxed paper and lightly spray with nonstick spray. Mound 30 small clusters of pecans, about 3 or 4 pecans each, spaced a couple inches apart on the pan.
Make caramel: Warm the cream over low heat and keep warm while you cook the sugar.
Put the sugar and corn syrup and in a deep, heavy-bottomed large saucepan. Cook over medium heat, stirring occasionally until the sugar dissolves. Stop stirring, raise heat to medium-high, and simmer until the sugar reaches the hard crack stage, or 305 degrees F on a candy thermometer, about 7 minutes.
Whisk the butter and salt into the sugar mixture. Gradually pour in the cream and vanilla taking care since the mixture will bubble up. Reduce the heat to medium and continue to cook, stirring occasionally, until the sugar reaches soft ball stage, 240 degrees F on the thermometer, about 5 minutes more. Immediately remove from the heat and cool for a minute.
Ladle a couple tablespoons of warm caramel over some of the nut clusters, to make the spider bodies. Then press 6 pieces of licorice into the warm caramel to make the legs. Repeat with the remaining caramel and licorice. (It's helpful to have an extra hand here, since the caramel can set quickly. If caramel hardens, warm over very low heat. ) Let spiders cool 15 minutes.
Meanwhile, put the chocolates in a medium heatproof bowl. Bring a saucepan filled with 1-inch or so of water to a very slow simmer; set the bowl over, but not touching, the water. Stir the chocolate occasionally until melted and smooth. (Alternatively, put the chocolate in a medium microwave-safe bowl. Melt at 50 percent power in the microwave until soft, about 1 minute. Stir, and continue heat until completely melted, 2 to 3 minutes more.)
Spoon about 1 tablespoon of melted chocolate on top of each spider. Sprinkle with sprinkles or chocolate curls, if desired. Let cool until firm.
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