Line a 9×9-inch pan with parchment paper or foil, leaving an overhang to lift the bars later. Lightly spray the lined pan with cooking spray. Set aside.

In a very large pot, melt 5 tablespoons unsalted butter over medium-low heat, stirring occasionally. When the butter is fully melted, reduce the heat to low.

Add most of the 1 (16-ounce) bag miniature marshmallows (the bag is divided; reserve the remainder to add later). Stir constantly over low heat until the marshmallows are just melted and the mixture is smooth.

Remove the pot from heat and stir in 1/4 teaspoon salt and 1 teaspoon vanilla extract until combined.

Add 6 cups Rice Krispies cereal to the melted marshmallow mixture. Stir quickly and thoroughly until the cereal is evenly coated.

Add the reserved miniature marshmallows from the bag to the pot and stir until they are incorporated and the mixture is uniform. Do not overmix.

Pour the mixture into the prepared pan. Lightly spray your hands with cooking spray (or grease them) and gently press the mixture into an even layer in the pan, pressing only enough to compact slightly—avoid pressing too hard to keep the treats soft and chewy.

Let the pan sit at room temperature until the treats have set, about 30 minutes.

While the treats cool, place 1/3 cup white chocolate chips and 1/4 teaspoon vegetable oil in a microwave-safe bowl. Microwave in 15-second bursts, stirring between each burst, until the chocolate is melted and smooth.

Transfer the melted white chocolate to a resealable plastic bag, snip off a corner, and drizzle the chocolate evenly over the cooled bars. Immediately add sprinkles, if using. Allow the chocolate to harden at room temperature.

Use the parchment overhang to lift the bars from the pan. For cleaner slices, run a sharp knife under hot water, dry it, and cut the bars. These treats are best enjoyed the same day.