Step 1: Prepare the Donut Dough : In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, and nutmeg. In another bowl, whisk buttermilk, eggs, melted butter, and vanilla. Combine wet and dry ingredients until a soft dough forms — it should be slightly sticky but easy to handle.
Step 2: Roll and Cut the Donuts : Lightly flour your work surface. Roll the dough to about ½-inch thickness. Use a donut cutter (or two round cutters) to cut donuts and holes.
Step 3: Heat the Oil : Fill a deep pan or Dutch oven with 2 inches of oil. Heat over medium heat until the oil reaches 350°F (175°C).
Step 4: Fry the Donuts : Fry 2–3 donuts at a time, about 1–2 minutes per side, until golden brown. Remove with a slotted spoon and place on paper towels to drain excess oil. Let them cool slightly before glazing.
Step 5: Make the Chocolate Frosting : In a bowl, whisk together powdered sugar, cocoa powder, and salt. Add melted butter, vanilla, and warm milk gradually, whisking until smooth and glossy. The glaze should be thick yet pourable — adjust with a little more milk if too stiff.
Step 6: Frost the Donuts : Dip the top of each donut into the chocolate glaze, allowing excess to drip off. Place on a wire rack. Immediately top with sprinkles before the glaze sets.