Wet Ingredients: In a large mixing bowl, use an electric mixer to beat together the butter, peanut butter, sugars, egg, and vanilla extract until well-mixed.
Dry Ingredients: In a medium mixing bowl, whisk together the gluten-free flour, baking soda, and salt.
Make the dough: Add the flour mixture to the butter mixture and mix until smooth. Then place the dough in the refrigerator for 30 minutes to cool. If refrigerating for longer, let rest at room temperature for 30 minutes before rolling into balls.
Shape and roll in sugar: Preheat the oven to 350 degrees F. Then use a melon baller or tablespoon measure to scoop heaping tablespooons of dough onto an ungreased cookie sheet. Leave about 2 inches between the dough balls to account for spreading. Then roll each dough ball gently with your hands until smooth before rolling it in the extra granulated sugar.
Mark and bake: Gently press the tines of a fork in a criss-cross pattern across the top of each cookie, compressing by about ⅓ of the original height of the dough ball. Bake for 9-12 minutes or until the outer edges are just slightly golden and the center appears set. Allow the cookies to cool for 10 minutes on the pan before moving to a cooling rack.
Cool the cookies and prepare the chocolate: Place the baked and cooled gluten-free peanut butter cookies in the freezer for 30 minutes. In the last 10 minutes, add the chocolate and vegetable oil to a large microwavable liquid measuring cup or jar with wide opening (I use a 2 cup liquid measuring cup). Microwave the chocolate in 20 second increments until completely melted and smooth. Stir together and let rest for 5 minutes before dipping. Meanwhile, line two baking sheets with parchment paper, wax paper, or a silicone liner.
Dip the cookies: Remove the gluten-free peanut butter cookies from the freezer. One at a time, dip each cookie about half way into the chocolate. Allow some of the extra chocolate to drip back into the mixture and lay the cookie flat on the lined baking sheet so that there is a little space between cookies. When all cookies have been dipped, place in the refrigerator until set. This usually takes about 20-30 minutes.