Coat a 13x9-inch baking pan with cooking spray, set aside.
Melt the butter and peanut butter in a large stockpot over low heat. Once the butter has melted, add the entire package of marshmallows. Cook, stirring, until the marshmallows, have melted. Remove from the heat and stir in the vanilla extract.
Immediately stir in the Cheerios until they are all coated. Transfer the mixture to the prepared baking pan. Using a spatula or your hand, firmly press the mixture into an even layer. Allow the treats to cool to room temperature.
Once the treats have cooled, make the chocolate topping. In the microwave or a double boiler, melt the chocolate chips. Once melted, stir in the avocado oil. Pour the chocolate over the treats and smooth into one even layer.
Allow the chocolate topping to set for at least 2 hours before cutting into slices.
Store bars in an airtight container at room temperature for up to 3 days.