Preheat your oven to 350°F.

Grease well the wells of two 24-cup miniature muffin tins, and set them aside.

Place the miniature peanut butter cups on a baking sheet and place in the freezer while you make the dough and then bake the cookies.

In a large bowl, place the flour, xanthan gum, cornstarch, baking soda and salt, and whisk to combine well. In a separate, medium-sized bowl, place the butter and peanut butter, and mix well until very smooth. Add the granulated sugar, light brown sugar, vanilla, and then the egg and egg yolk, mixing to combine after each addition. Create a well in the center of the dry ingredients and add the wet ingredients. Mix to combine. The dough will be thick but soft.

Divide the dough into balls, each about 2 teaspoonsful, rolled tightly into 1-inch balls. Place the balls in the center of the prepared wells of the miniature muffin tins. If the dough seems too difficult to handle, chill it in the refrigerator, covered, until it firms up and is easier to handle.

Place the muffin tins in the center of the preheated oven and bake until very lightly golden brown and just set in the center (the dough should no longer be shiny), about 9 minutes. Remove from the oven and immediately press a frozen peanut butter cup into the center of the cookie in each well, and press down until the top of the peanut butter cup is flush with the top of the cookie/well. Allow the cookies to cool fully in the muffin tin before using a toothpick to pry each one gently from its well.