In a large bowl, whisk together the flour, brown sugar, baking powder, and salt. Cut in the pieces of butter using a fork or pastry cutter until it forms coarse crumbs.
In a measuring cup or bowl, whisk the sweet potato with the buttermilk. Stir into the flour mixture until the mixture is moistened and forms a soft dough.
If your dough is very wet, you can rest it in the fridge for 10 minutes. Then, turn out the dough onto a surface floured with gluten-free flour, and knead it 4 times. Pat the dough into 1-inch thickness and use a biscuit cutter to cut out 1-inch biscuits. You can also form the dough into 12 equal-sized pieces. Place the biscuits 1 inch apart on a parchment-lined baking sheet.
Preheat the oven to 425℉ and place the whole tray of biscuits in the freezer to chill while the oven heats. Then bake for 20-25 minutes, until golden brown. Gently transfer the baked biscuits to a wire rack, and let them cool for 5-10 minutes until they are set. Serve the biscuits warm.
Notes
How to make sweet potato puree: You can either peel and cut a large sweet potato into chunks and boil it, or roast a whole sweet potato. To use the stovetop method, boil the chunks in a pot of water until tender. Then drain, mash, and cool the potato before using. To use the oven method: Scrub a sweet potato and prick it with a fork. Bake it at 400F until soft, about 1 hour. Let the potato cool, then cut it open and scoop out the insides to mash.
Use cold ingredients. Keep the butter and buttermilk in the fridge until you're ready to add them to the dough. This will help the biscuits be extra flaky after they've baked.
Handle the dough gently. Gluten-free dough is a bit easier to work with than regular dough because you don't have to worry about activating the gluten in the wheat flour. But with biscuits, too much kneading of any type of flour can lead to a dense result. So gently stir and fold it together.
Make ahead: Put together the dough and form the biscuits, then transfer the formed biscuits to an airtight container. Store the formed biscuits in the fridge for up to 2 days, or keep them in the freezer for up to 3 months.
Make them dairy-free: To make these biscuits dairy-free, substitute your favorite non-dairy butter for the butter and use non-dairy milk for the buttermilk.
Make them refined sugar-free: Swap the brown sugar and granulated sugar for coconut sugar to make these sweet potato biscuits refined sugar-free.