Line a baking sheet with parchment paper or a silicone baking mat. Add the chocolate chips and vegetable oil to a microwave safe bowl and microwave in 30 second increments, stirring in between, until completely melted and smooth.
Pour half the chocolate on your prepared baking pan and spread into a rectangle that’s roughly 9×13 inches. Transfer to the fridge or freezer to firm up for 15 minutes.
Reserve 1 cup of mini marshmallows for later, then in a saucepan over medium heat, combine butter and marshmallows. Stir continuously until melted and smooth, then add in the vanilla extract and stir again to incorporate.
Fold in the rice krispie cereal, remaining mini marshmallows, and sprinkles.
Evenly press the rice krispie mixture on top of the chilled chocolate layer. Spread the remaining chocolate on top, and optionally garnish with more rainbow sprinkles.
Transfer to the fridge or freezer to chill for another 15 minutes, then break or cut into pieces before enjoying.