Lightly coat a 9x13-inch baking pan with nonstick spray; set aside.
In a large pot, gently melt butter over low heat to prevent browning.
Once melted, add the mini marshmallows.
As marshmallows start to melt, stir in the peanut butter. Continue stirring constantly until completely melted and smoothly combined.
Take off the heat.
Fold in the Rice Krispies cereal.
Press the cereal mixture gently and evenly into the prepared pan.
For the topping, place chocolate chips in a microwave-safe dish.
Microwave for 30 seconds, then stir.
Stir in coconut oil, then continue microwaving and stirring in 15-second bursts until smooth.
Spread this melted chocolate evenly over the bars.
Allow to set for 30 minutes, or until firm, before slicing and decorating.
Remove from pan, cut into squares, and set aside.
To create spiders, microwave candy melts at 50% power for 30 seconds. Stir. Repeat in 30-second intervals until completely smooth.
Transfer melted candy to a piping bag with a small tip.
Place a mini Reese's peanut butter cup in the center of each square.
Pipe "legs" from the peanut butter cup outwards using the piping bag.
Dab two small dots of melted chocolate on the front of the peanut butter cup, then gently press on the candy eyes.
Let decorations set and cool completely before serving.