Instructions

for the rice krispies treats

Make parchment sling for 13 by 9-inch baking pan by folding 2 long sheets of parchment paper; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of parchment in pan perpendicular to each other, with extra parchment hanging over edges of pan. Push parchment into corners and up sides of pan, smoothing parchment flush to pan. Grease parchment.
Melt 8 tablespoons unsalted butter in Dutch oven over medium-low heat. Stir in 2 teaspoons vanilla extract, 1½ teaspoons salt, and 1 teaspoon cinnamon and cook until fragrant, about 30 seconds. Increase heat to medium, add 16 ounces large marshmallows, and cook, stirring frequently, until marshmallows are just melted, 2 to 4 minutes (some marshmallows may not be fully melted; this is OK).
Off heat, stir in 8 cups cereal until fully combined. Transfer cereal mixture to prepared pan. Using greased spatula, press cereal mixture into even layer. Let sit for 30 minutes until set.

for the spicy chocolate sauce

Meanwhile, heat ⅔ cup sugar, ⅓ cup whole milk, ⅛ teaspoon salt, ⅛ teaspoon ground cinnamon, and ⅛ teaspoon chipotle chile powder in small saucepan over medium-low heat, whisking until sugar is dissolved and liquid starts to bubble around edges of saucepan, about 4 minutes. Reduce heat to low, add 2½ tablespoons unsweetened cocoa powder, and whisk until smooth.
Off heat, stir in 1½ ounces finely chopped unsweetened chocolate and let sit for 3 minutes. Whisk sauce until smooth and chocolate is fully melted. Whisk in 2 tablespoons cubed and chilled butter and ½ teaspoon vanilla until fully incorporated and sauce is thickened slightly. Cover saucepan and set aside until ready to serve, whisking to recombine before serving. (Sauce can be refrigerated for up to 3 days; gently warm in microwave, stirring every 10 seconds, until pourable, before using.)
Combine ½ cup sugar and remaining ¾ teaspoon ground cinnamon in shallow dish. Use parchment sling to lift bars out of pan; discard parchment. Cut treats in half lengthwise, then slice crosswise into 1-inch wide strips. Working with a few bars at a time, roll in cinnamon-sugar mixture then transfer to serving platter. Serve with warm chocolate sauce. (Bars can be stored at room temperature for up to 3 days; toss in cinnamon-sugar mixture just before serving.)