Our double-chocolate tart is decorated with petroglyphs, or cave paintings. Create your own by cutting figures out of cardboard for stencils.
Heat oven to 375 degrees. On lightly flouredplastic wrap, roll chocolate dough into a6-by-16-inch rectangle 1/8-inch thick. Liftplastic wrap; flip dough over 4-by-14-inchtart tin. Press dough intopan, pressing into corners of tin andagainst fluted edge, and trimming flush with edge. Refrigerate at least 30 minutes.
Prick bottom of crust in several places with fork. Blind-bake crust: Line crust with foil that hangs over edges; fill with dried beans. Bake 20 minutes; let stand 5 minutes. Lift foil and beans from crust.
In a medium pan over medium-high heat, cook cream, milk, and sugar until cream and milk are scalded, stirring to dissolve sugar. Place chocolate in medium metal bowl. Pour scalded mixture over chocolate; whisk gently. Add eggs; whisk gently. Add extracts; whisk gently.
Pour chocolate filling into crust. Bake until filling has set, 30 to 35 minutes. Let cool in pan on wire rack.
Using small sifter, sift 2 tablespoons cocoa over tarts. Combine remaining 2 tablespoons cocoa with confectioners' sugar. Set stencils over tart. Sift sugar mixture over tart and remove stencils.