How to Make It
Preheat oven to 350°. Line two rimmed baking sheets with parchment paper. Trace three (4 in.) circles on each parchment sheet (a cereal bowl or ramekin is a handy guide). Turn paper over.
In a medium bowl, whisk together egg white, sugar, vanilla, and salt. Whisk in half of flour, then melted butter. Add remaining flour and whisk until smooth (batter will be thick and a little sticky). Spoon 1 1/2 tbsp. batter into center of each traced circle and, with an offset spatula, spread evenly to fill tracing.
Put one pan of cookies in oven and bake until a light, dappled golden brown, 9 to 12 minutes. Put second pan in 5 minutes after starting first pan. Meanwhile, wash and dry offset spatula. Set book next to oven, spine facing up and jacket spread a little to stabilize book.
Remove first pan from oven. Working quickly, use spatula to remove cookies and fold them, bottom sides up, over spine of book to form into taco shapes. Hold in place until firm, about 20 seconds (use hot pads if you find this uncomfortable). Lift taco shells off book and set on a plate. Repeat with second pan.
Remove ice cream from freezer and let soften slightly. Set out a small high-sided pan such as an 8- by 8-in. baking pan.
Working with 1 shell at a time, carefully spoon ice cream into shell (they break easily). Fill as full as possible, ideally flush with top. Set in pan and slide into freezer while you fill the next shell.
Freeze 30 minutes, then add more ice cream to each shell if necessary (ice cream may have deflated). Freeze 1 hour more.
Pour Chocolate Shell into a medium bowl to a depth of at least 1 1/2 in. and set peanuts within reach. Working with 1 taco at a time, dip ice cream side down into Chocolate Shell to coat just beyond rims (have a wet kitchen towel handy to wipe chocolate off your fingers). Invert taco and sprinkle with peanuts before chocolate sets.
Return dipped taco to freezer; repeat with remaining tacos. Freeze tacos at least 30 minutes to harden completely.
*If ice cream is airy, you may need as much as 4 cups, because it will collapse when measured.
Make ahead: Shells, up to 1 day, stored uncovered inside a turned-off oven. If they soften, re-crisp in a 350° oven for 3 minutes, draped over rungs of oven rack so they retain their shapes; let cool. Filled tacos, up to 2 weeks, frozen airtight.