Sift the sugar, cocoa powder, cornstarch, and salt into a medium bowl. If there appear to be any lumps, sift until the lumps have disappeared. Pour the dry ingredients into a multi-cooker and whisk in the milk, vanilla, and chocolate. Set the multi-cooker to the “white rice” program and bring the mixture to a boil, whisking constantly. Cook uncovered, until the pudding begins to thicken, 3 to 5 minutes.

Remove the bowl from the multi-cooker. Working quickly, pour or spoon the pudding into six 4-ounce ramekins or cups. Allow them to set up in the refrigerator, about 2 hours. The puddings can be covered and refrigerated for up to 2 days.

Melt the butter in a medium sauté pan over medium heat. Add the graham cracker crumbles and toast for 2 to 3 minutes, or just until golden brown.

To toast the marshmallows, preheat the broiler. Place the marshmallows on a lightly greased baking sheet, and broil them for 30 seconds. Using tongs, carefully turn the marshmallows over and broil for another 15 seconds. Remove from the oven and set aside. (Alternatively, you can toast them over a gas burner; see Note.)

When you’re ready to serve the pudding, spoon about 1 tablespoon of the toasted graham cracker crumbles on top of each serving. Place 2 or 3 toasted marshmallows on top of the graham crackers, and serve immediately.

Note : To toast the marshmallows on a gas stove, place 2 or 3 marshmallows on each of six long wooden or metal skewers, far enough down so that they will not easily fall into the fire when roasting. Place the skewers of marshmallows close to the flame of a gas burner for a charred result, or hover them above it for a more golden brown. Turn the skewers slowly, cooking all sides of the marshmallows.