Preheat oven to 350°F (180°C), or 160°C for a fan oven, and line a muffin baking pan with 12 cupcake liners.
In a medium sized bowl, whisk together the eggs, sugar, oil, and vanilla extract.
Stir in milk, then add the rest of the ingredients (flour, cocoa powder, baking powder, salt), and mix until just combined but do not over mix. Mix in peanut butter. The batter won't be very thick, sort of runny and that's okay.
Pour the batter into liners, and bake for 18-20 minutes or until toothpick comes out clean. When completely chilled, frost the cupcakes.
To make frosting, combine all ingredients in a mixing bowl and beat with a mixer.
To make spiderwebs and spiders, download and print out this template. Melt white chocolate in a double boiler - a bowl placed on a saucepan with simmering water until completely melted. Transfer the melted white chocolate to a pipping bag (with Wilton tip 3), pipe out spiderweb shapes on the template covered with parchment paper, transfer to the fridge for 5 minutes to harden.
To make the spiders, melt dark chocolate then pipe out spiders directly onto the spiderwebs (or use black icing). When they are set, decorate cupcake frosting with them.