For the crust:
For the filling:
To make the crust, combine the flour, cocoa powder, and salt in a small bowl. Whisk to blend. In the bowl of an electric mixer, beat the butter and sugar on medium-high speed until light and fluffy, 2-3 minutes. Blend in the egg and vanilla. With the mixer on low speed, add in the dry ingredients alternating with the cream, beginning and ending with the flour mixture. The dough will be quite soft. Transfer the dough to a piece of plastic wrap, form into a rectangle, and wrap tightly. Chill until firm, at least 1 hour and up to 2 days.
Transfer the chilled dough to a well floured work surface. Roll the dough out into a 7 x 16-inch rectangle and fit into a 4½ x 14-inch rectangular tart pan, pressing the dough into the edges of the pan.* Trim away any excess. Prick all over the surface of the dough with the tines of a fork. Freeze the lined tart pan until the dough is firm, about 30 minutes.
Preheat the oven to 350˚ F. Line the crust with parchment paper and fill with baking beads, dried beans or rice. Bake until the edges are set, about 20 minutes. Remove the weights and parchment and bake 5-10 minutes more. Let cool completely, about 30 minutes.
To make the filling, place ½ cup of the peanut butter in a small microwave-safe bowl. Heat until fluid, about 20-30 seconds. Pour into the bottom of the tart crust and smooth into an even layer. Freeze until the peanut butter is firm, about 20 minutes. Place the chopped chocolate in a medium bowl. Bring the cream just to a boil. Pour over the chocolate and let sit 1-2 minutes. Meanwhile, heat the remaining ¼ cup of peanut butter until fluid. Transfer to a plastic bag. Whisk the chocolate mixture together until a smooth ganache forms. Pour the ganache into the tart shell in an even layer. Snip a small tip off of the corner of the plastic bag and pipe the peanut butter in diagonal lines on top of the ganache. Use a toothpick or skewer to marble them gently together. Chill until well set, at least 1 hour and up to 8 hours. Slice and serve.
*While this tart dough recipe can certainly be used in other shapes and sizes of tart pans, this is the only size I have tried. You may need to increase the dough (or not) to fill pans of other sizes. If you try it with another size, please let me know what adjustments (if any) you made so we can share the love.