How to Make It
In a standing electric mixer fitted with the paddle, beat the butter and sugar at medium speed until fluffy. Beat in the egg. Sift the flour, cocoa and salt into the bowl and beat at low speed until the dough comes together, 2 minutes. Turn the dough out onto a lightly floured surface and knead 2 or 3 times. Flatten into 2 disks, wrap in plastic and chill until firm, about 30 minutes.
Preheat the oven to 350°. Line 2 large baking sheets with parchment paper. Roll out each disk of dough between 2 sheets of plastic wrap to a scant 1/4-inch thickness. Using a floured 1 3/4-inch round cookie cutter, stamp out as many rounds as possible; transfer to the baking sheets. Gather the scraps, reroll and stamp out as many cookies as possible. Bake the cookies for about 20 minutes, until puffed and set, shifting the baking sheets for even baking. Let the cookies cool on the baking sheets.
Oil the bottom and sides of a 9-inch-square cake pan and line the bottom with a sheet of wax paper, long enough to reach 2 inches up 2 sides of the pan. In a medium saucepan, stir together the sugar, cream, honey, corn syrup and vanilla seeds and bring to a boil. (Reserve the vanilla pod for another use.) Set a candy thermometer in the saucepan and cook over moderately high heat, stirring occasionally, until the temperature reaches 250°. Remove from the heat. Remove the thermometer and stir in the butter. Pour the caramel into the prepared pan and let cool for 45 minutes.
Lift the caramel out of the pan and transfer to a cutting board. Using a 1 1/4-inch round cookie cutter, stamp out rounds of caramel and sandwich them between the cookies, pressing gently to flatten slightly. Wrap any remaining caramel bits individually in wax paper and save for another use.
Melt the chocolate with the butter in a double boiler. Line a baking sheet with wax paper. Dip the cookies halfway into the chocolate glaze, allowing any excess to drip back into the pot. Set the cookies on the wax paper and refrigerate for about 20 minutes, until just set. Transfer the cookies to a plate and serve.