The key to the marbleized look of this chocolate-peanut butter tart: a wooden skewer, which is used to swirl warm peanut butter atop chocolate ganache.
Make the crust: Whisk flour, salt, and cocoa powder. Beat butter and sugar with a mixer on medium-high speed until pale and fluffy, about 3 minutes. Add egg and vanilla, and beat until combined, scraping down sides of bowl. Reduce speed to low, and gradually beat in flour mixture in 3 batches, alternating with cream and beginning and ending with flour mixture. Shape dough into a rectangle, wrap in plastic wrap, and refrigerate until firm, at least 1 hour and up to 2 days. Let stand at room temperature 5 minutes before rolling.
Roll out dough into a 7-by-16-inch rectangle on a floured surface. Fit into a 4 1/2-by-14-inch rectangular tart ring on a parchment-lined baking sheet. Push dough into corners of ring to create sharp edges; trim so that dough is flush with top of ring. Prick dough all over with a fork. Freeze until firm, about 30 minutes.
Preheat oven to 350 degrees. Line crust with parchment, pressing so that it is flush with dough, especially in corners. Fill with pie weights or dried beans; bake until edges are set, about 20 minutes. Remove weights and parchment; bake until cooked through, about 10 minutes. Let cool completely, about 30 minutes.
Make the brittle: Bring sugar, butter, corn syrup, and water to a boil in a small saucepan over high heat. Cook, stirring occasionally, until mixture begins to brown. Reduce heat to medium, and cook until golden brown. Stir in baking soda, salt, and peanuts until well combined (mixture should be the color of peanut butter). Immediately pour onto a nonstick baking mat, spreading out as thinly as possible with a heatproof spatula or wooden spoon. Let cool completely, about 30 minutes. Finely chop half the brittle with a heavy knife to yield 1/2 cup. Reserve remainder for another use.
Make the filling: Heat peanut butter in a microwave until fluid, about 30 seconds. Pour 1/2 cup into crust, and top with chopped brittle, pressing it into peanut butter. Freeze tart until peanut butter is firm, about 10 minutes. For ganache, bring cream to a boil; pour over chocolate in a small bowl. Add salt; let sit 8 minutes.
Meanwhile, fill a disposable pastry bag with remaining fluid peanut butter (if peanut butter has thickened, microwave it 10 seconds), and snip off end. Whisk ganache until smooth, and immediately pour into tart, smoothing with a mini offset spatula if necessary. Pipe peanut butter in continuous diagonal zigzag lines down length of tart. Draw a wooden skewer perpendicularly through peanut butter lines in a zigzag motion down length of tart. Then draw skewer in a continuous sideways loop pattern down length of tart to create swirls.
Refrigerate tart until set, at least 1 hour and up to 8 hours. Cut into wedges, and serve.