Place the yeast, milk, and half and half for the dough into your mixer. Mix to combine and let sit for 10 minutes.
Add the flour, sugar, salt, eggs, and melted butter. Mix to combine using the dough hook, and continue kneading for 3 - 4 minutes.
Remove from the mixer into a gallon baggie that's been lightly sprayed with cooking spray. Close the baggie and flatted out the dough. Place in the refrigerator to chill for 1-2 hours.
While the dough is chilling, make the pastry cream by whisking together the sugar, flour, salt, and egg yolks until creamy and smooth.
Warm the milk and half and half until it is steaming, but NOT boiling. Remove from heat, and SLOWLY stream in about 1/8 of a cup of the hot milk into the egg mixture while whisking the entire time. After it is mixed in, continue SLOWLY streaming the hot milk into the eggs while whisking until half of the milk has been mixed in. Now drizzle the egg/milk mixture into the sauce pan and mix into the remaining milk. Heat over medium heat, stirring constantly, until thick and glossy. Whisk in vanilla. Remove from heat and place into a bowl. Cover with plastic wrap with the wrap touching the pastry cream, and chill.
When the dough has finished chilling, remove from the fridge and roll out on a well-floured surface until its about 1/2 inch thick. Cut out into circles, and place onto a parchment lined baking sheet. (Save your scraps for oil temperature testing!)
Cover with plastic wrap, or a tea towel, and place into your proofing oven for about an hour. (If your oven doesn't have a "proof" setting, just turn it on to 350 for a minute or two, and then shut off. You want it to be about 100 degrees in there. Not too hot though!)
When the proofing time is nearly over, preheat your oil over medium heat. Test the temperature by frying a bit of your scraps. They should slowly brown, in about 3ish minutes. When the oil is the correct temperature, fry your donuts in small batches, and remove them from the oil directly to the sugar and toss to coat. Place on paper towels to cool completely.
Mix in marshmallow fluff to the pastry cream, and place into a piping bag fitted with a filling tip like the one pictured in the post above. Pipe into cooled donuts and serve immediately.