For the crust:
Preheat the oven to 350°F. Spray 5 (4 1/2-inch) tart pans with nonstick cooking spray.
In the bowl of a food processor pulse the Oreo cookies until finely ground. Add the butter and pulse until incorporated. Divide the crust mixture among the tart pans and press into the bottom and up the sides of each pan. Bake for 8 minutes, or until the crust is set. Let cool completely.
For the filling:
Combine the water and gelatin in a measuring cup or small bowl. Stir until dissolved.
In a medium saucepan combine the sugar, cocoa, cornstarch, and salt. Gradually add the milk, whisking until smooth. Place over medium heat and cook, whisking constantly, until the mixture comes to a boil and thickens, about 10 minutes.
Remove from heat and add the chocolate, stirring until melted. Stir in the vanilla and the gelatin mixture. Divide the pudding between the prepared tart pans and chill in the refrigerator until set, 2 to 3 hours or overnight.
For the topping:
In a small saucepan combine the sugar, corn syrup, water, and salt over medium-high heat. Boil, stirring occasionally, until the mixture reaches 240°F on a candy thermometer.
Meanwhile, beat the egg whites and cream of tartar with an electric mixer fitted with a whisk attachment. Whip on medium-high speed until soft peaks form.
With the mixer on low speed, slowly drizzle the syrup down the side of the mixing bowl. Increase the speed to medium-high and beat until stiff and glossy peaks form and the mixture is no longer hot, about 7 to 9 minutes. Add the vanilla during the last minute of beating.
Spoon the marshmallow crème among the chilled tarts. With a kitchen torch, lightly toast the marshmallow. Serve.