Prepare a 9x13 baking pan by spraying it with nonstick spray and adding parchment paper to the bottom for easy removal.
In a large pot, melt the butter over medium-low heat. Once the butter is almost completely melted, add the marshmallows.
Stir to coat the marshmallows fully in the butter. Cook for 5 to 8 minutes, stirring intermittently until the marshmallows melt completely. Don't overcook, as this will cause the mixture to burn and the marshmallows to harden.
Remove the pot from the heat. Stir in vanilla extract and salt.
Pour in the cereal and crushed graham crackers. Stir to fully combine, making sure to scrape any additional marshmallows from the bottom of the pot.
Stir in additional 1 ½ cups of mini marshmallows.
Dump out the cereal and marshmallow mixture into the 9x13 baking pan. Using a greased spoon, spread out evenly from edge to edge.
Melt chocolate chips and coconut oil together in a medium bowl. Whisk until smooth. Pour over the top of the rice krispie treats and smooth with a spatula until they're fully covered in melted chocolate.
Allow the rice krispie treats to cool and the chocolate to harden for about 15 minutes before slicing into 15 even-sized rectangles.