Proof yeast by stirring the warm water, sugar, and yeast in a small bowl. Leave to rise and bubble for about 5 minutes before using. If yeast does not foam or bubble, it is no longer active and should be discarded.
Heat milk and butter in a microwave- safe bowl for about 40 seconds, or until the milk and butter has reached about 110°F (use a thermometer)
In your stand mixer with the dough hook, combine all yeast mixture, milk mixture, and all remaining ingredients and mix on low until it all comes together. If dough is still quite sticky, add 1/4 cup more flour.
Knead on medium speed for about 5 minutes, or until the dough is tacky to the tourch. Do not continue kneading past this point or the donuts will become like bread.
In a large greased bowl, add the dough and cover with a clean towel. Trasnfer to a warm location. I like to heat the oven to 250°F, then turn it off. This provides the perfect location for your dough to rise.
After dough has tripled in size, punch dough down and transfer dough onto a floured surface.
Roll dough until about 1/2" thick. Use a heart-shaped (or circle) cookie cutter to cut out donuts. Transfer each donut carefully to a large parchment lined sheet pan. Cover with a clean towel, then transfer to a warmed oven to rise again, this should take about 45-60 minutes.
In a dutch oven, add oil and heat until 350°F. Use a candy thermometer to keep track of the temperature and keep it consistent.
Fry donuts 1-2 minutes per side or until each side is golden brown, then flip.
Use a large slotted spoon to transfer the donuts to a cooling rack.
While they're still warm, coat with sugar if desired.
Fill with Chocolate-Hazelnut Spread. We like ChocZero, which contains no sugar.