For the gluten-free flour blend: In a large bowl, whisk together the rice flour, tapioca flour, potato starch, xanthan gum and salt.
For the cookies: In a small bowl, whisk together the cocoa, baking soda, cinnamon, ground ginger, salt, chopped chocolate and 1/4 cup candied ginger with 1 1/2 cups gluten-free flour blend.
Preheat the oven to 350 degrees F and line 2 baking sheets with parchment paper. Beat together the oil and sugar with an electric mixer on medium speed until the sugar is dissolved. Add the eggs and molasses; mix until combined. With the motor running, gradually add in the flour mixture and mix until combined.
Place some sugar in a small bowl. Using a 1 1/2-inch ice cream scoop, drop balls of dough into the sugar; toss to coat and place on the prepared baking sheets. Bake until puffed and cracked on the tops, about 15 minutes. Place on wire racks and let cool completely.
Dip the cookies into the melted white chocolate and decorate with candied ginger. Place on parchment paper until completely set.