Step 1

In a small bowl combine the yeast and warm water; set aside until it foams up, about 5 minutes.

Step 2

In the bowl of a stand mixer fitted with the hook attachment, combine the warm milk and butter and, using a silicon whisk, stir until the butter has completely dissolved. Add in the sugar, egg, egg yolk, salt, yeast mixture and 3 cups flour, and beat on medium-low speed until smooth. Beat in remaining flour just until a soft ball of dough forms around the hook. Don't knead the dough yet!

Step 3

Transfer the dough to a large, lightly greased bowl, turning once to coat the top. Cover tightly with saran wrap and set aside to rise in a warm area until it appears to have tripled in size, about 1 hour.

Step 4

Gently punch the dough down and turn it out onto a lightly floured work surface. Kead the dough into a smooth ball, about 7-8 times, or until it's uniform in texture.

Step 5

Using a rolling pin, roll the dough out into a large rectangle, approximately 1/2 inch thick. Using a lightly floured 2 and 1/2-inch round dough cutter, cut an equal amount of circles from the rectangle.

Step 6

Transfer the cut doughnuts to a large, lightly greased baking sheet, spacing them an inch a part (they will rise). Cover pan lightly with saran wrap and set aside for 45 minutes, or until the doughnuts have almost doubled in size.

Step 7

When ready to fry, heat oil in a large, heavy bottomed pot, to 360°(F). Fry doughnuts, a few at a time, for 1-2 minutes on each side, or until golden brown.

Step 8

Quickly roll the warm doughnuts in the cinnamon sugar and set aside.

Step 9

Add chopped chocolate to a large, heatproof bowl and set it aside until needed.

Step 10

In a small saucepan over medium-heat, bring the cream to a low simmer. Pour half of the cream on top of the chopped chocolate and set aside for 1 minute. Using a silicon whisk, begin to incorporate the melted chocolate into the cream. Slowly add the remaining cream and continue whisking until the mixture is completely smooth and glossy.

Step 11

Add in the butter, salt, and liqueur (if using), and use a silicon spatula to stir until the butter is completely melted. The mixture should be smooth, dark and glossy.

Step 12

Cool to room temperature, then place in the fridge to chill until thick but still smooth enough to pipe, about 2 hours.

Step 13

In a medium-sized shallow bowl, combine the granulated sugar and cinnamon and set aside until needed.

Step 14

Scrape the chocolate hazelnut cream into a pastry bag fitted with a star tip and set aside.

Step 15

Using the back of a small spoon (or your finger) make a small hole on the side of each doughnut.

Step 16

Carefully fit the nozzle of the bag into a doughnut and squeeze until you see the center begin to puff up. Repeat with all doughnuts.