Prepare a 9x9" baking pan by greasing it generously with canola oil (or another neutral-flavored cooking oil) or butter. Set aside.
Drain the maraschino cherries (save the syrup for another use if you'd like) and spread on a paper towel-lined plate. Gently pat dry with a second paper towel. You should have approximately 3/4 cup of cherries. Transfer the cherries to a food processor (a mini one is best for this small job) and pulse a few times until the cherries are coarsely chopped. If you don't have a food processor, coarsely chop with a large knife. Set the chopped cherries aside.
In a large microwave-safe bowl (or a large saucepan), melt 4 Tablespoons unsalted butter (about a minute on high). Add 1/4 teaspoon salt, 10 ounces mini marshmallows, and the chopped maraschino cherries. Microwave on high for 30 second intervals until the marshmallows are almost fully melted, stirring between each interval (it took me a total of 90 seconds in the microwave).
Stir 1/2 teaspoon vanilla extract into the marshmallow mixture, then pour in 6 cups rice krispies. Stir until all of the rice krispies are evenly coated with marshmallow. Spread the rice krispie mixture into the prepared baking pan, and press gently with a silicone spatula until flat. Sprinkle red and/or pink sprinkles over the top of the marshmallow mixture, if desired. Let sit at room temperature until the marshmallow is completely set, about 30-60 minutes.
Use a large chef's knife to cut into squares (remove the whole slab of rice krispie squares from the pan first, and cut on a cutting board to avoid damaging your pan and knife). Serve immediately or store in an airtight container or zipper bag at room temperature for up to 2 days. Can be stored for a longer period of time in the freezer. Thaw at room temperature before eating.