Preheat a waffle iron. In a large bowl, whisk together the Biscuit Blend, powdered buttermilk, sugar, salt, and baking soda. If the buttermilk powder has become the least bit clumpy, press it through a sieve using the back of a spoon. If it's silken smooth, just whisking the dry ingredients together is sufficient.
In a separate bowl, whisk together the sour cream, eggs, oil, and vanilla until they are smooth. Using a wooden spoon, stir the sour cream mixture into the dry mixture until you have a uniform batter. Whisk in the seltzer. From here, move fast so the seltzer doesn't lose its bubbles.
Brush a bit of the melted butter onto the plates of the hot waffle iron. Ladle about 1/4 cup (60 ml) batter onto the iron to fill the plates three-quarters full. Close the waffle iron and cook according to the manufacturer's instructions. (Alternatively, watch the steam escaping from the iron. Once the steam has diminished to a bare vapor, the waffle is ready.) The cook time for most waffle irons is 3 to 5 minutes, and the finished waffle should be golden brown and release easily from the plates of the iron. Repeat with the remaining batter.
Serve the waffles immediately, hot off the iron.
Try substituting 1/4 cup (26 g) tapioca starch for the Biscuit Blend for an even lighter version.
"The Everyday Art of Gluten-Free" by Karen Morgan. Copyright Stewart, Tabori & Chang 2014. Provided courtesy of Karen Morgan. All rights reserved.