Prepare the chocolate cake according to the package directions in two 8-inch cake pans and let cool completely.
Spray an 8-inch bowl with cooking spray. Line it with plastic wrap as neatly as possible and spray the wrap with cooking spray. Put in the freezer until very cold.
Line a baking sheet with parchment and spray with cooking spray. Microwave the chocolate in a medium microwave-safe bowl in 30-second intervals, stirring in between each, until melted, about 3 minutes. Let cool for 5 minutes.
Cut off 1 inch from the chocolate ends of 8 biscuit sticks, so you have 8 short and 8 regular-size sticks total. In same-length pairs, dip the chocolate ends into the melted chocolate and lay them on the prepared baking sheet so that they overlap slightly in the shape of a V. Refrigerate until set.
Stir the coconut oil into the remaining chocolate.
Remove the prepared bowl from the freezer and brush the inside with 1/4 cup of the chocolate, then refreeze until firm, about 10 minutes. Repeat 4 more times, each time making sure the chocolate goes all the way to the rim of the bowl and doesn't pool in the bottom. Freeze until solid, about 30 minutes.
Put one cake layer in the bowl of a food processor and pulse to fine crumbs. Add the frosting and pulse until it just comes together. Form into a ball that is slightly flattened on the bottom where it will sit on the platter, and freeze for 30 minutes. Remove from the freezer and brush to coat with the chocolate mixture. Refreeze for 15 minutes, then repeat 2 more times. On the final time, affix the eyes to the ball in the shape of a horseshoe. Attach a small round piping tip (1S or 2) to the tube of red decorating icing, and outline each eye with icing.
Fill the chocolate-lined bowl with the marshmallows. Top with the remaining cake layer and then flip it out onto a large serving platter. Remove the plastic wrap from the chocolate dome. Arrange the head so it touches the chocolate dome body. Drizzle the body with the remaining chocolate mixture and sprinkle with the chocolate sprinkles. Make a black widow marking by piping a red hourglass on the upper third of the body close to the head. Sprinkle it with red sanding sugar. Position 4 biscuit stick legs on each side of the body, making sure the 2 shorter ones go in the middle; gently push the legs into the dome about 1 inch.
When ready to serve, heat the strawberry topping in a small saucepan over medium heat until it boils. Slowly pour it over the spider's body to reveal all the eggs inside. Then run, screaming for your life.
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