For the streusel, I used 1/2 c. of white rice flour and 1/4 c. of sweet rice flour to replace the AP flour. My streusel was a little dry, and I think if I were to do it again, I'd increase the melted butter by another tablespoon. Other than that, I used Abby's directions.
To make the cake gluten free, I replaced the AP flour with the following mixture:
1/2 c. amaranth flour
1/2 c. sweet rice flour
1/4 c. tapioca starch
1/4 c. potato starch
3/4 c. white rice flour
1 tsp. xanthan gum
I also increased the baking soda to 1-1/2 tsp, as gluten free baked goods sometimes need a bit of extra oomph.
Then I got creative.
I replaced the sour cream with a 6 oz. container of vanilla Greek yogurt and 1/2 c. of canned pumpkin. I beat the pumpkin into the butter/sugar/egg mixture, and folded in the yogurt as instructed for the sour cream. I also increased the vanilla extract to 1 tablespoon - in part because I was using a homemade vanilla extract and it's a little young, and in part because I wanted to embrace the whole "pumpkin pie" vibe.
You can see a bit of what I mean about the streusel being slightly dry. I also pulled it out just a smidgen too early, as it was just a little underdone in the middle. But still tasty, on a warm fall afternoon:
Both of my children devoured their pieces, and even my husband remarked that it didn't TASTE gluten free. I'd call that a Score! all the way around. Thanks, Abby!!