Top of Gluten-free cupcakes crunchy?

I'm on a mission to find a vanilla gluten free cupcake recipe. I've run about 10 experiments so far. Variations of Annalise Roberts recipe. It uses sugar, eggs, brown rice flour, potato flour, tapocia starch, salt, baking powder, xanthan gum, canola oil/palm oil, rice milk/coconut milk, and vanilla.  I've tested a various temps, baking times, resting times, with various milk substitutes, tested oil vs. shortening.  Regardless of the variables I've introduced, the tops of the cupcakes are crunchy, and a tad bit sandy. Can anyone out there with knowledge of gluten-free baking help me figure out what ingredient is the cause of this. The inside of the cake if fine. 

I also noticed it, in the samples I picked up earlier in the week from various local bakeshops, but didn't think anything of it at the time. 

Is this the norm? Anything I can do to overcome it? 

The only thought I have is too put a pan of water in the bottom of the oven. Which is going to be my next experiment. I'm also going to try replacing part of the brown rice flour with coconut flour and increasing the amount of liquid.

Thanks in advance for your help!

Quote:

Originally Posted by jason_kraft 

I recommend white rice flour instead of brown rice flour, and sorghum flour instead of potato flour.

Thanks! I have a recipe with those ingredients listed so I give it a shot! 

Quote:

Originally Posted by tawnyadee 

I like Gluten-Free Pantry brand all-purpose flour.  Maybe you could just substitute that in your regular vanilla cupcake recipe.

I just looked it up, and they use white rice flour also. Thanks! I'm going to try a scratch recipe with white rice flour and see where that gets me, since I have a ton to use up. If that fails, then I'll try again next weekend with a pre-blend as you suggested if I have a light order week. 

All this for a dear friend who hasn't eaten cake in years because she was so disappointed in gluten-free things she's tried. She thinks I can come up with something she likes so I've made it a goal to come up with something for her by the end of the year. 

Made a rookie mistake tried a few recipes with Sweet White Rice Flour and it was a gummy mess. Apparently there is White Rice Flour and Sweet White Rice Flour. Guess I'll continue my mad science experiment when I get the right White Rice Flour (tongue twister).  I need to check  my potato starch too. The one I has says Potato Starch Flour, how ambiguous is that?

Go ahead, laugh at me!