For the tart shells, cream together the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment until smooth and lightly aerated. Sift together the flour and cocoa powder, and mix into the butter. Whisk the egg yolks together and add to the bowl, mixing just until incorporated.
Form the dough into a flat rectangle and wrap tightly in plastic wrap. Place in the refrigerator and let rest for at least 1 hour.
Transfer the dough to a lightly floured surface and roll out to 1/2 inch thick (you should have a square of about 12 inches). Brush away any excess flour and, using a 4 1/4-inch round cutter, cut out 4 circles. Place four 3 1/4-inch by 1/2-inch-high ring molds (or tartlet pans) on a baking sheet lined with parchment paper or Silpat. Line each ring mold with a circle, pressing the dough into the base of each ring to ensure there are no air pockets. Trim off any excess dough. Refrigerate for 30 minutes before baking.
Preheat the oven to 300°F. Bake the tart shells on the middle rack of the oven for 7 to 8 minutes. Rotate the baking sheet and bake the shells for an additional 7 to 8 minutes, until golden brown. Remove the baking sheet from the oven and allow the tart shells to cool on a rack before unmolding them, then store in an airtight container until ready to use.
To assemble the tarts, place the baked tart shells on a clean parchment-paper-lined baking sheet. Sprinkle 1 tablespoon of the chopped peanuts over the bottom of each tart. Spoon 1 1/2 teaspoons of the caramel sauce (gently warmed if necessary) into each shell, covering the peanuts. Place the tarts in the freezer for 15 minutes to allow the caramel to set slightly, so it can be cleanly topped with the ganache.
Remove the tarts from the freezer and spread 1 tablespoon of the chocolate ganache evenly into each one. Allow the tarts to stand at room temperature for at least 30 minutes before serving, to allow the ganache time to set. (The finished tarts can be kept at room temperature for up to 8 hours before serving.)
