1. Sift flour, cocoa, and 3/4 teaspoon salt into a medium bowl. Beat butter and sugar with a mixer on medium speed until pale and fluffy, about 4 minutes. Beat in egg and vanilla. Reduce speed to low, and add flour mixture in 3 additions, alternating with heavy cream, beginning and ending with flour. Shape dough into a rectangle, and wrap in plastic wrap. Refrigerate for 30 minutes.

2. Roll out dough between 2 sheets of lightly floured parchment to a 1/16-inch thickness. Refrigerate until firm, 15 to 20 minutes.

3. Preheat oven to 350 degrees. Working in batches, line twenty-four 3-inch barquette molds (1/2 inch deep) and twenty-four 2 1/4-inch round tartlet molds: Cut out dough 3/4 inch wider than each shape on all sides, fit inside mold, and trim excess by rolling a rolling pin over tops. Place a matching mold over dough in each tart shell and press down gently. Freeze until tart shells are firm, about 20 minutes.

4. Bake tart shells on baking sheets for 15 minutes with top molds still in place. Remove top molds and bake until tart shells are cooked through, 7 to 10 minutes more. Let cool completely in molds set on wire racks. Remove tart shells from molds.

5. Transfer jam to a disposable pastry bag and snip a 1/4-inch opening. Pipe about 1 teaspoon jam evenly into the bottom of each tart shell.

6. Transfer ganache to another disposable pastry bag or a resealable plastic bag and snip a 1/4-inch opening. Fill each tart shell. Let stand until set, about 2 1/2 hours at room temperature or 1 hour in the refrigerator. (Refrigerating tarts will make ganache set faster but be less shiny.)