Line an 8×8-inch baking pan with parchment paper and lightly grease it.
In a large saucepan over low heat, melt the butter.
Add the marshmallows and stir continuously until fully melted and smooth.
Remove from heat and immediately fold in the Rice Krispies cereal until evenly coated.
Transfer the mixture to the prepared pan and gently press it into an even layer without compacting it too firmly.
Allow the Rice Krispy treats to cool completely, then cut into bars or squares.
Melt the chocolate in a microwave-safe bowl in 20–30 second intervals, stirring between each, until smooth. Stir in oil if needed for a thinner consistency.
Dip one end of each Rice Krispy treat into the melted chocolate, letting excess drip off.
Place dipped treats on parchment paper and add toppings if desired.
Let the chocolate set completely at room temperature before serving or storing.