How to Make It
Stir together brown sugar, baking powder, salt, and nutmeg in a medium bowl. Whisk together buttermilk, butter, and egg in a small bowl; add to sugar mixture, and whisk to combine. Stir in 1 cup of the flour until blended. Stir in 1 more cup of the flour until blended. If dough is too sticky to handle, stir in remaining ¼ cup flour, 1 tablespoon at a time, until just dry enough to handle.
Cover dough with plastic wrap and chill at least 1 hour or up to 3 hours.
Pour oil to a depth of 1 inch in a large cast-iron skillet and fit with a deep-fry thermometer; heat to 350°F over medium-high.
Turn dough out onto a lightly floured surface, and roll to ½-inch thickness. Using a 3-inch round cutter, cut out 8 rounds; use a 1-inch round cutter to cut out holes in the center.
Working with a few at a time, and being sure not to overcrowd the skillet, gently lower donuts into hot oil. Fry, turning once, until golden brown and done, 2 to 3 minutes per side. Transfer to a paper towel-lined baking sheet to drain, and let cool completely.
Stir together powdered sugar, vanilla, and 1 tablespoon of the milk in a small bowl. Add remaining 1 tablespoon milk, 1 teaspoon at a time, if you prefer a thinner glaze. Dip one side of donuts into glaze, and sprinkle with candy sprinkles and place on a parchment lined sheet until set.
Coffee Glazed Donuts: Prepare recipe as directed substituting 1 to 2 tablespoons strong brewed coffee for the whole milk in the glaze and chocolate sprinkles for the rainbow ones.