Mix flour, sugar, salt, and yeast in a bowl.
Add eggs, warm milk, and melted butter.
Knead until elastic by hand or in mixer. Cover and let rise for 1 hour.
Punch down the dough and divide into 65g portions.
Roll each piece into a smooth ball and place onto individual squares of baking paper.
Arrange on a tray, cover loosely, and let rise for another hour.
Heat milk and vanilla until steaming.
In a bowl, whisk cocoa, eggs, yolks, sugar, cornstarch, and salt.
Slowly add the hot milk while whisking constantly, then return to pan and cook until thick & bubbling.
Off heat, stir in chocolate and butter.
Chill with plastic wrap touching the surface.
Fry donuts in hot oil.
Fry until golden.
Drain and roll in sugar.
Transfer the chilled chocolate pastry cream to a piping bag fitted with a round nozzle.
Poke a small hole in the side of each donut and pipe in the filling generously.
