Coat a 9x13 inch pan lightly with butter or non-stick cooking spray. A thin, even coating helps the finished slab release without tearing at the corners.

Place the butter, mini marshmallows, and peanut butter in a large saucepan over low heat, stirring gently as they warm. Stop heating when the mixture becomes a puffy, glossy ribbon with no distinct marshmallow lumps remaining.

Take the saucepan off the heat and immediately fold in the Rice Krispie cereal. The cereal should become evenly coated, with faint crackling as the warm marshmallow mixture reaches each piece.

Transfer the cereal mixture to the prepared pan and guide it into an even layer with lightly greased hands. Use only enough pressure to fill the corners because a firmly packed layer bakes into dense, tough squares.

Put the chocolate chips in a medium-sized heatproof bowl and microwave in 45-second intervals at medium power, stirring after every interval. The chips should look mostly melted before the final stirring, with a few soft pieces still visible.

Stir the chocolate until completely smooth, then blend in the vegetable oil. The topping should look glossy and flow easily from the spoon rather than sit in a thick lump.

Spread the chocolate over the cereal layer and leave the pan at room temperature for about 2 hours. The chocolate is ready when it holds a soft sheen and no longer shifts when the pan is gently moved.

Slice the slab into squares and place them in an airtight container at room temperature. Use a sharp knife and wipe the blade between cuts if the chocolate begins to drag.