Make the cookie dough
Preheat your oven to 325°F
Line rimmed baking sheets with unbleached parchment paper and set them aside
In a small, heavy-bottom saucepan, place 1 cup peanut butter and 8 tablespoons butter
Melt, stirring frequently, over medium heat
Once the mixture is melted and smooth, remove from the heat and set aside to cool briefly
In a large bowl, place the flour, xanthan gum, baking powder, baking soda, salt, granulated sugar and brown sugar, whisking to combine well and working out any lumps in the brown sugar
Add the melted peanut butter and butter mixture, and stir to combine
Add the egg and vanilla, and mix until the dough comes together
Knead until smooth and flatten into a disk
Shape the dough
Place the cookie dough between two sheets of unbleached parchment paper, and roll out into a rectangle about ¼ inch thick
Cut out oval shapes with a 2-inch oval cookie cutter (Ateco sells a set of concentric oval shaped cookie cutters)
Pull away the scraps of uncut dough, gather the scraps and set them aside
Carefully transfer the ovals to the prepared baking sheets, placing them about 1 inch apart (they will not spread much during baking)
With a moistened thumb and forefinger, carefully pinch each oval on either side of the width of each oval’s center to create a peanut shape
Re-roll the gathered scraps of dough and repeat the process, reserving about ¼ cup of dough Place the reserved dough in a small bowl and mix with hot water by the tablespoon and stir until you have a thick paste
Transfer the paste to a pastry bag fitted with a #2 tip, and pipe a crisscross pattern on half of the cut-outs
Bake the cookies
Place the baking sheets in the freezer until the cut-outs are firm (about 5 minutes), and then place each, one at a time, in the center of the preheated oven and bake, rotating once, for 9 minutes or until the cookies are lightly golden brown all over
Remove from the oven and allow to cool completely on the baking sheets
Prepare the filling
While the cookies are cooling, heat the 4 tablespoons unsalted butter and ¾ cup peanut butter in a small, heavy-bottom saucepan over medium heat, stirring frequently, until just melted
Remove the pan from the heat and add the dash of kosher salt and stir to combine. Add the confectioners’ sugar, and stir until well-combined
The mixture will be very thick
Add one tablespoon of cream, and stir to thin the filling
Add another tablespoon of cream if necessary to create a thickly pourable filling
Allow the filling to cool until no longer hot to the touch
Assemble the cookies
Place the filling in a pastry bag fitted with a medium-sized plain tip (I used a #12 tip)
Turn over the half of the cookies without the crisscross pattern, and pipe a generous amount of filling on each overturned cookie
Top with the decorated cookies to create sandwiches, pressing down gently to force the filling to the very edge of the cookies
Allow to sit at room temperature (or in the refrigerator) until the filling is set
Serve chilled or at room temperature