Preheat the oven to 175C (350F) and line 2x12-hole shallow muffin tins with cake liners (bake the final 6 as a separate batch unless you have a third muffin tin).
Whisk the vegetable oil and caster sugar together in a large bowl until creamy (about 2 minutes with an electric whisk). Add the eggs, vanilla extract and buttermilk and stir to combine.
Sieve in the cocoa powder, flour, bicarbonate of soda and salt. Fold in using a spatula. The mixture will be runny. Spoon the mixture into the cupcake liners - approx ⅔rds full.
Place in the oven and bake for 13 minutes, until an inserted skewer comes out clean. Leave to cool for 10 minutes in the tray, then transfer to a baking rack to cool completely.
Now make the frosting. Place the butter and peanut butter in a bowl and whisk together using an electric hand whisk until creamy.
Add the confectioners' sugar, cream, vanilla extract and salt and whisk again for 2-3 minutes until thick. If it's too runny, add a little more confectioner's sugar until you get the right consistency for piping.
Spoon the mixture into a piping bag with a wide round nozzle. Pipe the mixture on the cakes, starting from the outside and working inwards.
Decorate each cupcake with 3 pieces of Snickers. Drizzle over the salted caramel and melted chocolate (you can drizzle on via a piping bag with a very small round nozzle if you want a neater drizzle).