Lightly grease a 9x13 inch pan with butter or non-stick cooking spray.

In a large saucepan melt the butter, mini marshmallows and peanut butter together over low heat while stirring gently.

Once melted, remove from the heat and stir in the Rice Krispies.

Press the mixture into the pan (lightly spray your hands with non-stick cooking spray to avoid the mixture sticking to your hands).

Add the chocolate chips (or chopped chocolate) to a medium-sized heatproof bowl. Microwave for 45-second intervals on medium power, stirring in between each burst.

Once almost melted, stir until smooth and stir in the vegetable oil.

Pour the chocolate over the top of the bars and let harden at room temperature (about 2 hours).

Cut into squares and store in an airtight container at room temperature.