Combine flour, sugar, cocoa and salt in medium bowl. Add butter and rub in, using fingertips, until mixture resembles coarse meal. add water and egg yolk and mix in with fork until well incorporated. Gather dough into rectangle chill 20 minutes. (Can be prepared 4 days ahead. Let dough soften slightly at room temperature before rolling out.)
Preheat oven to 375°F. Butter and flour 13 3/4 x 4-inch rectangular pan with 1-inch-high sides and removable bottom. Roll out dough between 2 sheets of waxed paper to 15x6-inch rectangle. Peel off 1 waxed paper sheet. Invert dough into prepared pan, press evenly to fit. Freeze until firm, about 10 minutes. Peel off second waxed paper sheet. Trim edges.
Line crust with foil; fill with dried beans or pie weights. Bake until crust is set, about 12 minutes. Remove foil and beans. Bake until crust just begins to darken around edges, piercing with toothpick if crust bubbles, about 12 minutes longer. Remove from oven; maintain oven temperature. Spread jam over bottom of crust. Bake until jam is set, about 3 minutes. Transfer pan to rack; cool completely.
Bring cream to boil in heavy small saucepan. Remove from heat. Add chocolate and whisk until melted and smooth. Transfer chocolate ganache to bowl and refrigerate until chilled but not firm, about 45 minutes or less
Using electric mixer, beat ganache until very thick and semifirm. Spread ganache over jam in crust. (Can be prepared 8 hours ahead. Refrigerate.) Arrange raspberries atop ganache. Stir powdered sugar and cocoa in bowl. Sift over tart.