Special equipment: a squeeze bottle or pastry bag and a skewer, optional
For the web: Put a large round shallow bowl on a larger round platter. Fill the bowl with the guacamole and smooth the top with a spoon or offset spatula. Fill a squeeze bottle or pastry bag with Mexican crema or sour cream (thin it out with some lime juice if needed). Pipe 4 concentric circles of crema onto the guacamole. Using the tip of a knife or skewer, pull lines from the center circle out, creating a spider web pattern.
For the spider: Slice 1 olive in half lengthwise and use one of the halves for the spider body. Slice 8 long spider legs from the remaining 1 1/2 olives. Arrange the spider on the web. Surround the bowl of guacamole with the tortilla chips.
For the skull: Using a paring knife, make 2 eyes on the wide end of the avocado, cutting through the skin and flesh all the way down to the pit; discard the skin and flesh. Cut an upside-down heart shape just through the skin for the nose, then remove the skin, leaving the flesh showing. For the mouth, cut a long rectangle just through the skin and remove it. Use a skewer or the tip of a knife to draw lines on the flesh for teeth.
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