Mix 1/4 t salt, plus chocolate and 2 cups confectioners' sugar in a bowl. Bring cream and 2 tbsp. water to a boil in a 1-qt. saucepan over high heat. Pour cream over chocolate; let sit for 2 minutes without stirring, then whisk until smooth. Dip tops of donuts in warm glaze and return to wire rack until set. Whisk vanilla extract and confectioners' sugar with milk in a bowl until smooth. Drizzle over chocolate glaze with a fork in a zigzag formation. Return donuts to rack until glaze is set.