Preheat your oven to 325°F. Line rimmed baking sheets with unbleached parchment paper and set them aside.

In a small, heavy-bottomed saucepan, heat the butter and peanut butter over medium heat, stirring frequently, until just melted. Set the pan aside to cool briefly.

In a large bowl, place the flour, xanthan gum, baking powder, baking soda, salt and granulated sugar, and whisk to combine well. Add the brown sugar and oats, and mix to combine, working out any lumps in the brown sugar.

Create a well in the center of the dry ingredients and add the melted butter and peanut butter, vanilla, and egg, and mix to combine. The dough will be thick but soft.

Using a spoon, wet hands or a spring-loaded ice cream scoop, scoop the dough into 24 portions, each about 2 tablespoons.

Place each of the portions of dough on the prepared baking sheets, about 2 inches apart from one another. Using wet hands, roll each portion of dough into a ball and press into a flat disk.

Place the baking sheets with the cookie dough on them in the refrigerator for about 10 minutes or the freeze for just a couple of minutes. This will help keep the cookies from spreading too much in the oven.

Once the dough has been chilled briefly, place the baking sheets, one at a time, in the center of the preheated oven and bake until the cookies are lightly golden brown all over and slightly darker around the edges (10 to 12 minutes, depending upon size).

Remove the baking sheets from the oven and allow the cookies to cool on the baking sheet until set (at least 10 minutes). Transfer to a wire rack to cool completely.

In a small, heavy-bottomed saucepan or a medium-size microwave safe bowl, place the butter and peanut butter and heat until just melted either over medium heat in the saucepan (stirring frequently) or in the microwave for about 45 seconds.

Remove the pan from the heat or the bowl from the microwave, and add the vanilla, salt and confectioners’ sugar, and stir until well-combined. The mixture will be very thick.

Add 1 tablespoon of the cream, and stir to thin the filling. It should be glossy. Add up to another tablespoon of cream, if necessary, to create a very thickly pourable filling.

Allow it to cool, and then transfer it to a piping bag fitted with a 1/4-inch plain pastry tip.

Once the cookies are completely cool, invert half of the cookies and pipe a generous amount of filling on each inverted cookie.

Top with the remaining cookies and press gently to sandwich. Allow to sit at room temperature or in the refrigerator until the filling is set before serving.