To make the Rice Krispies treats, grease an 8x8-inch pan with cooking spray or brush with olive oil. Place the olive oil, tahini and sweetened condensed milk in a medium pot on the stove and melt over medium heat, stirring occasionally with a heat-proof flexible spatula. Add the bag of marshmallows and stir until melted and smooth. Stir in the vanilla and salt.
Off the heat, add the Rice Krispies and the remaining one cup marshmallows, if using, stirring until the cereal is coated and everything is combined. Transfer to the prepared pan, drape with a sheet of plastic wrap and press down lightly to evenly flatten the treats (you do not want to pack the Krispies treats down too tightly, or they will lose their inherently soft and gooey texture).
To make the glaze, combine all of the ingredients in a medium microwave safe bowl. Microwave on high in 50 second bursts, stirring after each, until melted and smooth. Alternatively, you can do this on the stovetop over medium heat in a small saucepan, stirring frequently. Pour the topping over the bars, then use a small offset spatula or butter knife to spread into an even layer. Chill in the refrigerator until set, about 1 hour.
Serve cold or at room temperature sprinkled with flaky sea salt, if desired. Bars will keep up to a week in the refridgerator, covered in an airtight container.