Gluten free flour plus a mixture of whipped eggs and sugar help keep the texture both light and fudgy at the same time. Makes sure to use a really good cocoa powder. I haven't tested with natural yet. So far I've only made these with Rodelle brand. For the chips I've tested with Guittard semisweet and Nestle's new "dark" chips.
I've also made these with Bob's Red Mill All-Purpose Gluten Free Blend. That flour is a little fluffier and requires about 1/3 cup plus 3 tablespoons to make the requisite 2.1 oz. If you are measuring by volume, stir the flour and measure as you would regular flour.