Chocolate lovers will adore these dark, rich tartlets. The silky bittersweet-chocolate filling provides a sleek backdrop for a rainbow of piped meringue hearts. Make a large version for a party or minis like these, each sized for two to share.
Preheat oven to 325 degrees. Make the tart shells: Coat five 5-inch heart-shaped tart pans with cooking spray. Whisk together flour, cocoa, and salt.
Beat butter with a mixer on medium speed until fluffy, about 3 minutes. Add sugar, and beat until pale and fluffy, about 2 minutes. Reduce speed to low. Add vanilla and flour mixture, and mix until dough just begins to hold together, 1 to 2 minutes. Turn out dough onto plastic wrap, wrap, and refrigerate for 30 minutes.
Roll out dough to 1/4-inch thickness on a lightly floured surface. Cut dough into 5 equal pieces, and press into bottom and up sides of prepared tart pans. Using a sharp paring knife, trim excess dough flush with edges of pans. Patch any tears. Refrigerate for 1 hour.
Bake tart shells on a rimmed baking sheet until firm, about 25 minutes. Let cool completely on wire racks.
Make the ganache filling: Place chocolate in a bowl. Heat cream and milk in a small saucepan over medium-high heat until almost boiling; pour over chocolate. Let stand for 2 minutes. Slowly whisk until smooth. Let stand 10 minutes more. Stir in egg. Return tart shells to baking sheet, and pour in filling. Bake until just set, about 25 minutes. Let cool completely before removing from pans. Decorate with meringue hearts.
Regardless of the pan size, baking times remain the same.