Preheat oven to 425°. Spray 15 (2½-inch) tartlet pans with cooking spray.
Unroll piecrust dough on a lightly floured surface. Using a 3¼-inch round cutter, cut 15 rounds from dough. Transfer each dough round to a prepared tartlet pan, pressing into bottom and up sides. Using the wide end of a chopstick, gently press dough into indentations in sides of pans. Trim excess dough. Using a fork, prick bottom of dough in tartlet pans. Place tartlet pans on a rimmed baking sheet. Freeze for 15 minutes.
Place a small piece of parchment paper in center of each prepared tartlet pan, letting ends extend over edges of pan, and place ceramic pie weights or dried beans on each parchment piece.
Bake until tartlet shells are set, 5 to 6 minutes. Carefully remove parchment paper and weights. Continue baking until tartlet shells are light golden brown, 3 to 4 minutes more. Let cool completely.
Reduce oven temperature to 350°.
In a large saucepan, whisk together egg yolks and sugar.
In a small bowl, whisk together cocoa powder, cornstarch, and salt. Add cocoa powder mixture to egg yolk mixture, whisking until incorporated. Gradually add milk, whisking until combined and smooth. Place saucepan over medium heat and cook, stirring constantly, until pudding comes to a boil. Boil, stirring constantly, for approximately 1 minute. (If at any point, pudding starts to stick to pan or scorch, turn down heat.) Remove pan from heat. Stir in butter and vanilla extract until incorporated. (Mixture should be thick and smooth.)
Strain pudding through a large, fine-mesh sieve into a large heatproof bowl. Divide pudding among tartlet shells.
Divide Meringue among tartlet shells, making sure Meringue completely covers chocolate mixture and touches tartlet shell edges to seal in chocolate mixture. Using a small spoon, create swoops and swirls in meringue.
Bake until Meringue peaks are lightly golden brown, 8 to 10 minutes. Transfer tartlets to a wire cooling rack. Let cool completely.
Notes
Make-Ahead Tip: Tartlets can be made up to a day in advance, stored in a covered container, and refrigerated.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chocolate-cream-tartlets/