Combine the flour, sugar, baking powder, baking soda, and salt in a food processor.
Drop the butter into the flour mixture in a few chunks. Pulse the food processor a few times to chop it up and disperse it evenly in small pieces, but don't run the machine much at this point.
Whisk together the egg and yogurt.
With the food processor running on low speed, pour the yogurt-egg mixture in. Stop when the dough forms a ball.
Allow the dough to sit for 30 minutes to absorb the moisture.
Preheat the oven to 500℉ (260℃). Line a baking sheet with parchment paper. Place a second baking sheet beneath it to double the sheets and keep your biscuits from burning on the bottom.
Roll out the dough on a floured surface until it's about ¾-inch (2 cm) thick. (I use plain white rice flour for this, rather than using up my all-purpose blend.) Cut biscuits into round shapes about 2-inches (5 cm) in diameter and place on the prepared pan.
Place the pan on the middle rack of the preheated oven, close the door, and then turn the heat down to 425℉ (218℃). Bake the biscuits for about 14 minutes, but keep an eye on them. Ovens vary. They should brown nicely on the top.
Remove the pan from the oven and place it on a cooling rack for a few minutes before enjoying your fresh, hot biscuits!
Notes
If you don't have a food processor, whisk the dry ingredients together by hand, cut in the butter, and stir the liquid mixture in well until the dough forms a ball. You may need to work it with your hands.
The 30 minutes of resting is a trick we have discovered for gluten-free baking to turn out better.
Don't lower the oven temperature until you have put the biscuits in to bake AND have closed the door. Otherwise, they won't get the blast of heat required for optimal rising.