In a small saucepan set over low heat, slightly warm the milk until it is lukewarm in temperature. Add in the yeast and whisk to combine. Let stand for 5 minutes.
In the bowl of a stand mixer, combine the flour, 1 tbsp. of sugar and 1 tsp. of sea salt.
Add in the yeast mixture followed by the eggs and mix on a low speed until the dough begins to come together, 2 to 3 minutes.
Increase the speed to medium-high and work the dough until it is very smooth, 4 to 5 more minutes.
Reduce the mixer speed to low then slowly add in the butter, a little at a time, over the course of 3 to 4 minutes – pausing half-way to scrape down the back of the bowl and hook.When the butter begins to blend in, increase the mixer speed to medium-high to fully incorporate and bring the dough together, 4 to 6 minutes longer.
Form the dough into a ball and transfer it into a lightly greased bowl. Cover with a damp tea towel or plastic wrap and let it rise up in a warm spot for 2 hours or until doubled in size.
Once risen, turn the dough out onto a lightly floured work surface and lightly dust the top with flour.
Roll the dough out until it is ½ inch / 1.3 centimetres in thickness.
Using a biscuit cutter, or glass as a measure, cut out 2 ½ inch / 6. 3 centimeter circles, re-rolling and cutting until all the dough has been used up.
Gently press a truffle into the center of each circle, and wrap the dough tightly around it, pinching the seams to enclose.
Set the doughnuts on two lined baking trays, cover them with a damp tea towel and let the doughnuts rise again for up to an hour or until puffed.
Meanwhile, make the chocolate sugar coating by mixing 5 tbsp. of the caster sugar with the dutch processed cocoa powder and 1 tsp. of sea salt. Set aside until needed.
When ready to fry, heat about 2 inches / 5 centimetres of the oil in a heavy based saucepan until it reaches 350 F / 180 C.
Fry the doughnuts in batches, about 3-4 at a time, rolling them around with a slotted spoon to help them colour evenly, until golden brown all over, roughly 3-4 minutes.
Immediately remove and roll the fried doughnuts in the chocolate sugar mixture to coat them evenly then set them on paper towels to drain away any excess oil.
Repeat with remaining doughnuts and serve. They’re optimal eaten warm while their insides are still molten, though you can store them, covered at room temperature.