In a medium bowl, whisk together the sugar, flour and egg yolks.

In a small saucepan, bring the milk to a boil. Over a low-medium heat, whisk in the egg yolk mixture, stirring all the while until the mixture thickens and begins to boil gently.

Remove the saucepan from the heat. Stir in the salt, butter and vanilla. Cool and refrigerate until needed. Cover the surface of the custard directly with plastic wrap to prevent a skin from forming on top.

In a double boiler, melt the chocolate with the butter. Add the sugar and vanilla and stir until smooth. Add enough water to make a thin glaze.

Let the mixture cool for one minute, then drizzle it over the doughnuts or dip the tops of the doughnuts into the glaze. If the glaze gets too stiff, add more hot water to make it workable again.

Fry up the doughnuts in small balls. Once you can handle them, fill with custard filling and then drizzle with the chocolate glaze. Let set.