Preheat oven to 400°F.
In a medium bowl, whisk together 1/2 cup powdered sugar, 1.1 ounces all-purpose flour (about 1/4 cup), 3 tablespoons unsweetened cocoa, 1 teaspoon cornstarch, and 1/4 teaspoon salt until evenly combined.
Add 3 tablespoons egg whites, 1 teaspoon 2% reduced‑fat milk, and 1/4 teaspoon vanilla extract to the dry mixture; whisk until smooth and slightly runny.
Line a baking sheet with parchment paper or lightly coat it with cooking spray. On the underside of the parchment, trace four 5‑inch circles to use as placement guides (you can use a 5‑inch round cookie cutter or a cup).
Drop 1 tablespoon of batter onto each traced circle and spread the batter thinly and evenly to the edges of the circle with the back of a spoon.
Bake at 400°F for 6 minutes, or until the edges begin to brown. Remove the baking sheet from the oven.
Loosen each disc from the parchment with a spatula. Working quickly while the discs are pliable, drape each disc over two parallel wooden spoons (or another rounded support) to form a taco shape. Cool the shaped shells completely on a wire rack. Handle gently—the shells are delicate.
Repeat steps 5–7 with the remaining batter so you have a total of 8 shells.
Place 1/2 cup semisweet chocolate chips and 1 teaspoon canola oil in a microwave‑safe bowl. Microwave on HIGH for 1 minute, stirring after 30 seconds; then stir until the chocolate is fully melted and smooth.
Divide the 1/2 cup finely chopped unsalted dry‑roasted peanuts into two equal portions.
Using a small spoon, spread some of the melted chocolate on the top third of the outside of both sides of each cooled shell. Immediately press some of the first portion of chopped peanuts onto the chocolate so they adhere. (Work carefully to avoid breaking the shells.)
Spoon 1/3 cup vanilla low‑fat ice cream into each shell.
Drizzle the remaining melted chocolate over the ice cream and sprinkle with the remaining chopped peanuts.
Place the assembled chocolate tacos in the freezer for at least 30 minutes before serving to firm the shells and ice cream.