2cupspacked dark brown sugar(15 ounces; 425 grams)
3largelarge eggs(about 5 1/4 ounces; 150 grams, out of shell)
1 1/2teaspoonspure vanilla extract
1cupbuttermilk(8 ounces; 226 grams)
Preheat oven to 350 degrees F. Grease a 10-cup bundt pan with non-stick cooking spray. Set aside.
Whisk together white rice flour, cornstarch, sweet rice flour, baking powder, baking soda, xanthan gum, and salt.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter and sugar until it just begins to light. Add the eggs, one at a time. Scrape down the sides and bottom of the after the last addition and cream for an additional 30 seconds.
Add 1/3 of the dry ingredients. Add until just blended. Add 1/2 the buttermilk and the vanilla. Cream until combined. Repeat with remaining dry ingredients and buttermilk.
After adding the last addition of the dry ingredients, increase mixer speed to medium-high. Blend for 20 seconds. Batter should be fluffy.
Spoon into prepared pan. Bake for 35-40 minutes or until a cake tester inserted into the cake comes out clean.
Remove pan from the oven and place on a wire rack. Allow cake to cool in the pan for five minutes and then remove from the pan and place back on the wire rack to cool completely. (Allowing the cake to stay in the pan longer than five minutes might cause it to stick.)