For the shells: Set a large wooden spoon on the counter so that the handle hangs over the edge. Weight the other end of the spoon down with a heavy book or heavy pan; you'll drape the warm shells over the handle to form your tacos.
Place the egg whites, sugar, milk, vanilla and salt in a medium bowl and whisk together. Stir in the flour and butter until fully incorporated and the batter is smooth.
Place a medium nonstick skillet or cast-iron pan over low heat. Scoop 1 generous tablespoon of batter into the skillet and spread into a thin even layer with a small offset spatula. Spread the batter quickly to form a nice round that's roughly 5 inches in diameter. Cook until the underside of the disc is browned and set, 3 to 4 minutes. Flip and continue to cook for 1 to 2 minutes.
Lift the disc off the griddle and quickly and carefully lay it over the handle of the wooden spoon. Guide the shell with your hands to make sure the sides are even. This must be done quickly as the shell will begin to harden almost immediately after leaving the skillet. Hold the taco for a few seconds until it hardens and then leave it there to cool. Repeat with the remaining batter.
These taco shells are best eaten the day they are made, but leftovers can be stored in an airtight container for up to 2 days.
For the tacos: Bring a medium saucepan filled with a few inches water to a bare simmer. Place the chocolate in a medium heatproof bowl. Set the bowl over (not in) the simmering water. Cook until the chocolate is melted and smooth, stirring occasionally. (Alternately, place the chocolate in a medium microwave-safe bowl and microwave at 30-second intervals until smooth, stirring between intervals.)
Dip the edges of a shell in the melted chocolate and sprinkle the chocolate with peanuts. Repeat with the remaining shells and allow to cool.
Scoop some ice cream into the shells. Drizzle fudge sauce over the ice cream and sprinkle with some more chopped peanuts. Eat straight away, or place filled tacos in the freezer to enjoy later.