In a medium saucepan, melt butter on low heat. Slowly add both bags of marshmallows, stirring frequently. Add vanilla extract and continue heating until melted.

In a large mixing bowl, add Rice Krispies and candy corn. Spoon melted butter and marshmallow mixture into the bowl and mix until Rice Krispies and candy corn are thoroughly coated.

Spoon mixture into a greased 9x13 casserole dish and flatten (be sure not to apply too much pressure when flattening).

Place M&Ms on the surface of the Rice Krispie treats, pushing in slightly. Melt chocolate in the microwave in a small bowl. Once melted, add vegetable oil and stir. Drizzle warm, melted chocolate on top of the treats using a fork.

Refrigerate and cut into squares to serve.