1.To make the pastry cream: Heat milk in the microwave for about 30-40 seconds. In a separate heat-proof bowl, whisk egg yolks, sugar, cocoa powder, and corn starch. Slowly pour the warm milk into the egg mixture and whisk. Strain and pour the mixture into a small pot and place on medium heat. Continuously whisk until mixture becomes thick (less than 10 minutes). Take pot off heat and add in dark chocolate chips (optional), and as they start to melt, stir to combine. immediately transfer to a bowl and cover with plastic wrap—plastic wrap should directly touch the cream. Chill for at least 2-3 hours or overnight until cold.
To make the brioche dough: Heat milk in the microwave for about 30-40 seconds (~110F), then add in yeast and whisk well. Let it sit for about 5 minutes until foamy. Then add in sugar and whisk. Add in egg and whisk.
Add bread flour and salt to standing mixer bowl and add in the wet mixture from previous step. Mix on low with dough hook until the dough comes together and separates from the sides of the bowl. It should mainly be stuck on the hook and the bottom of the bowl (about 5 minutes).
Add in butter, one tablespoon at a time–mix on low until all incorporated. This can take some time, but it will get there!
Increase the speed to medium-high and knead dough for about 2-3 minutes until smooth and tacky
Form dough into a smooth ball. Place it into a large greased bowl (use 1/2 tsp oil), cover and let proof for 2 hours, until doubled in size.
After 2 hours, punch dough to deflate and remove excess air. On a lightly floured surface, divide dough into 9 equal pieces, roughly 70g each. Roll each piece of dough into a round ball, then roll out into a slightly flat round shape with a rolling pin (1/2 inch thick). Place each donut onto a lightly greased individual piece of parchment paper. Cover and let proof for another 1-2 hours until doubled in size.
Fry donuts: Heat a generous amount of oil in a pot to 350F. Fry donuts on each side for 1.5-2 minutes until each side is golden brown. Take out donuts and place on a wire rack. Let cool for 10-15 minutes
To coat donuts: Combine vanilla bean sugar and cinnamon in a bowl. Dip and coat each donut in the vanilla bean cinnamon sugar.
To fill donuts: Poke a hole in the white band of each donut using a chopstick or skewer. Take out chocolate pastry cream from fridge and mix with a spoon/spatula to soften the cream. Fill a piping bag with cream, and pipe cream into donuts.