Line an 8-inch square baking dish with parchment paper to overhang the edges by an inch.
Whisk the sugar and cocoa together in a medium saucepan. Whisk in the milk until smooth. Set the pot over medium heat, stirring to keep the mixture smooth, and bring to boil. Continue to boil, stirring the edges to keep from burning, until the mixture reaches 220 degrees F, about 60 seconds. (The mixture should be thick, smooth and glossy, not crystallized.) Remove from the heat and stir in the vanilla and salt, beating until smooth. Add the peanut butter and stir in, leaving pockets of peanut buttery goodness intact if desired. Let cool until warm but still stirrable, about 5 minutes. (You want the mixture to be soft enough that you can stir in the walnuts and marshmallows, but not so hot that the marshmallows melt entirely.)
Stir in the marshmallows and walnuts. Spread the mixture in the prepared pan and add the flakey salt and sprinkles. Let cool completely, about 1 hour. Cut into squares to serve. (To store, refrigerate in a covered container with parchment between the layers.)