1 1/4 cups buttermilk
1 cup mashed, oven roasted sweet potato, room temperature
1 teaspoon vanilla extract
4 cups self-rising soft-wheat flour (I used King Arthur unbleached Self-Rising Flour)
1 Tablespoon baking powder
1/8 teaspoon salt
1 cup unsalted butter (2 sticks), cut into 1/2-inch cubes and chilled
In a medium bowl, stir together the buttermilk, mashed sweet potato, and vanilla. In a large bowl, whisk together flour, baking powder, and salt. Cut butter into flour mixture with a pastry blender until it is crumbly and resembles small peas. (Alternatively, place the dry ingredients in the bowl of a food processor fitted with the blade attachment. Add the butter cubes and pulse until mixture is crumbly. Transfer the butter/flour mixture to a large bowl.) Add the buttermilk/sweet potatoes to the butter/flour mixture, folding the mixture together using a rubber spatula, just until the dry ingredients are moistened and the mixture comes together. Turn the mixture out onto a lightly floured surface. Use your hands to pat the dough into a rough square, about ½” thick. Fold the dough in half, then fold in half twice more. Pat the dough into a ¾” thickness. Cut circles using a flour dusted biscuit cutter, cutting straight down through the dough, without twisting the cutter. Place the biscuits on a parchment lined baking sheet. Cover with plastic wrap and refrigerate for 30 minutes.
In a medium bowl, whisk together ½ cup granulated sugar and ½ teaspoon cinnamon.
When ready to fry the doughnuts, fill a heavy bottomed pot with 2 inches of vegetable oil. Heat until the temperature reaches 360 degrees F on a candy thermometer. Place 4-5 doughnuts at a time into the hot oil, being careful to maintain the temperature. Fry for 3-4 minutes per side, until the doughnuts are deeply golden. (It’s a good idea to carefully remove one of the doughnuts with a slotted spoon and slice it open to make sure the center is well fried and not doughy.)
Drain on paper towels, blotting off excess oil, and toss in cinnamon sugar. Using a pastry bag or a squeeze bottle, fill each doughnut with blueberry filling.