Bloom the Yeast - Combine together active dry yeast, warm buttermilk and 1 teaspoon of sugar in a measuring cup or bowl. Allow to bloom for 8 to 10 minutes. The yeast mixture should be frothy.
Combine all Ingredients - In a large mixing bowl or stand mixer bowl, add all-purpose flour, bread flour, frothy yeast, remaining sugar, eggs, yolk, melted butter, vanilla, malted milk powder (if using) and salt.
Knead the Dough - Fit your stand mixer with the dough hook attachment and knead on medium/high speed for about 15 minutes or until it passes the window pane test. If it's a bit sticky, you can gradually add 1 to 2 tablespoons of bread flour. see notes below for kneading by hand and window pane test Let Double in Size - Form dough into a ball, then place into a lightly oiled bowl (I used the same stand mixer bowl). Cover with plastic wrap and allow to rise in a warm place for about an hour or until it doubles in size.
Divide the Dough - Gently punch your dough down to deflate it and then transfer to a lightly floured surface. Portion into 14 equal pieces. Using a kitchen scale, measure each ball of dough to make sure they are as even as possible. Mine were about 65 to 70 grams per ball.
Shape the Donuts - Grab a piece of dough and press it flat to get rid of any air bubbles. Gather and pinch the edges together and then place it on your work surface seam side down. Form your hand into a relaxed “claw” shape and cup it over the ball of dough. Then move your hand in small circular motions, gently dragging the ball along. It will tighten and the top will become smooth. Place each ball of dough onto a 4×4 inch (10×10 cm) piece of parchment paper. I placed 6 to 8 donuts on each baking tray to give them more than enough room to proof.
Proof the Donuts - Loosely cover with plastic wrap that has been sprayed with a bit of oil and allow to rise for about 45 minutes or until it passes the “poke” test. see not below Prep the Oil, Coating & Filling - With 20 minutes left of proofing, get your oil ready so that you can start frying right away. Fill a medium to large sized pot with at least 2 inches of vegetable shortening or neutral oil such as canola, vegetable, or avocado oil. During this time, you will also want to fill your Nutella in a piping bag or plastic zip bag, combine cinnamon sugar in a shallow bowl or dish and line 2 wire racks with paper towels.
Fry the Donuts - Once the oil reaches 350ºF to 360ºF (176ºC to 182ºC), pick up 2 to 3 donuts by the parchment paper and gently place it into the hot oil. After a few seconds, remove the parchment paper with tongs. Fry for 1 minute on each side and then flip and fry for an additional 45 seconds on each side. (set a timer).
Coat & Fill - Transfer cooked donuts onto a paper towel-lined cooling rack and allow to cool for a few minutes before rolling into your cinnamon sugar. Using a chopstick or small paring knife, create a small hole/pocket on the side of each donut and fill with as much Nutella as you'd like. Devour right away and enjoy!!