Liberally spray an 8” square baking pan with oil and set aside
In a large microwave-safe mixing bowl, heat the marshmallows and butter or oil on high for 90 seconds. Stir and if the marshmallow mixture doesn’t become smooth, heat for an additional 30 seconds
Immediately add the vanilla extract and Simple Mills chocolate cake mix and fold in until combined. It’s ok if the marshmallow isn’t all the way incorporated – the swirls of marshmallow are very pretty – but don’t stop mixing until you’re certain that there are no globs of cake mix left in the batter
Turn the batter out into the prepared pan and use a spatula to spread and flatten it into an even layer. It will be springy and take some coaxing, so be patient and gentle with it. If you need to, dampen your hands and use your palms to flatten the batter. Top with extra marshmallows if desired, and gently press them into the batter
If you’d like to garnish with a chocolate drizzle, melt together the dark chocolate and coconut oil (in a microwave-safe bowl) in 30 second increments in the microwave, stirring in between, until very smooth. Use a spoon to drizzle the warm chocolate over the tops of the bars. Don’t worry about it being perfect – this is the kind of thing where messy and rustic is better!
Refrigerate the bars for at least 1 hour (or as long as overnight) until completely cooled and firmed up. Use a very sharp knife to cut into bars