Make the donut dough. In a stand mixer fitted with the dough hook attachment, combine the bread flour, sugar, milk, egg, egg yolk, salt and instant yeast. Let the machine run for 10 minutes on low speed, until the dough comes together.

Cut the softened butter into cubes and slowly add the butter to the bread dough as the stand mixer runs. Increase the speed to medium-low and let the mixer run for around 15-20 minutes, until the dough is smooth, supple, and pulls away from the sides of the bowl.

Cover the bowl with cling film and let rise in the fridge overnight. Alternatively, cover the bowl and let the dough sit at room temperature until doubled in size, around 1-1.5 hours.

Make the chocolate pastry cream. Pour the milk and vanilla into a small saucepan over medium heat and stir until combined.

Meanwhile, whisk together the egg yolks, sugar, and cornstarch in a separate bowl.

When the milk comes to a slight simmer, take the saucepan off the heat. Whisking constantly, pour the hot milk into the egg and sugar mixture one ladle at a time. Pour the egg mixture back into the saucepan and place over medium-low heat.

Continue to whisk the milk mixture until it begins to thicken and bubble. Whisk for 20 seconds after the first bubble, then take the saucepan from the heat.

Add the chocolate and gently stir until it melts from the residual heat and is well combined. Pour the pastry cream into a bowl or spread it over a baking dish. Cover the cream with plastic wrap, pressing the plastic wrap directly to the surface of the pastry cream so it will not form a skin.

Place the cream into the fridge to cool for at least one hour.

Fry the donuts. Cut a piece of parchment paper into 12 squares (around 3 inches on both sides), and lightly dust them with flour.

Take the bread dough from the fridge and punch it down. Divide the dough into 12 pieces (around 60g each), and roll them into seamless balls.

Place the donuts onto the prepared parchment squares and let rise at room temperature for 30-45 minutes, until slightly puffed.

Line a cooling rack with paper towels and set aside. In a small bowl, mix together ⅔ cup granulated sugar with 2-3 teaspoons of cinnamon and set aside. Fill a large pot with vegetable oil (around 2 inches deep), and heat to 350°F.

Gently tip the top of the donuts into the oil, peeling back the parchment paper. Fry them two or three at a time for 1½ to 2 minutes on each side (be sure to check your oil temperature to make sure it doesn't get too high or low), then transfer the donuts to the prepared cooling rack.

Assemble the donuts. While still warm, coat the donuts in the prepared cinnamon sugar and let them cool slightly.

Take the pastry cream from the fridge and stir, making sure everything is well incorporated and there are no lumps. Transfer the cream to a piping bag. With a knife, pierce the side of the donuts to make room for the cream. Insert the piping bag into the donut, filling it with as much pastry cream as desired.

Serve immediately or store in the refrigerator for 3-4 days. Enjoy!