Prep. Preheat the oven to 200°F. Arrange two oven racks to divide the oven into thirds. Line two baking sheets with parchment paper and set aside.
Beat the egg whites. Add the egg whites to a large mixing bowl. Using a hand mixer or stand mixer on high speed, beat until they become foamy and start to hold their shape slightly.
Add the cream of tartar, sugar, and vanilla. Once the egg whites are foamy, add the cream of tartar and continue beating until soft peaks form. Then, gradually add the sugar, about 1 tablespoon at a time, beating well after each addition. Continue until the egg whites are glossy and hold stiff peaks. Be careful not to overbeat. Finally, add the vanilla and beat until incorporated.
Pipe and bake. Pipe or spoon 24 mounds (about 2 inches wide) onto the prepared baking sheets. Transfer to the oven and bake for 2 hours, rotating the pans halfway through to ensure even baking.
Let the meringues set. Turn the oven off and leave the meringues inside to cool for 40 minutes to 1 hour. They’re done when the surface feels dry, and they lift cleanly off the parchment without sticking to your fingers.
Transfer and serve. Remove the meringues from the oven and carefully transfer them from the parchment to a serving plate.
For the chocolate
Melt the chocolate. In a small microwave-safe bowl, combine the chopped chocolate chips and vegetable oil. Microwave for 40 seconds to 1 minute, stirring every 30 seconds until smooth and melted.
Dip the meringues. Dip half of each meringue into the melted chocolate. Let the excess drip off, then place the cookies on a baking sheet lined with wax paper.
Let the chocolate set before serving. Allow the dipped meringues to sit at room temperature until the chocolate is fully set. Don’t refrigerate. Once set, serve or store in an airtight container.