In a saucepan over low heat, warm milk, coconut sugar and butter until dissolved
Add dissolved mixture to a mixer with a dough hook, and 2 cups of flour. Make sure the milk mixture has cooled down enough before adding in your starter.
Add starter, eggs, the rest of the flour, and salt. Knead on low in the mixer until the dough is smooth and elastic. This takes about 10 minutes for the dough to finally come together and pull away from the sides.
Cover the bowl with plastic wrap and leave in a warm place to rise for 5-6 hours. The top of the fridge is my favorite place.
Place dough in the fridge overnight to ferment.
The next day, roll out on a lightly floured surface to 1/2″ thickness.
Cut the donuts out and place them on a sheet. You can use a biscuit cutter or just a wide mouth mason jar lid. To cut the centers I use a bottle cap. Make sure to save your donut holes!! They are the best!
Cover with a damp tea towel or plastic wrap for 1.5-2 hours to do a second rise.
Add about 1/2 inch of oil to a cast iron skillet. Coconut oil is my favorite for this.
Heat the frying oil over medium heat. You want it to be hot enough to sizzle when something is added to the hot oil.
Fry 2-3 minutes, flip, and fry another 2-3 minutes on the other side until puffy and golden brown. Be careful not to overcrowd the pan.
Place on a wire rack over parchment paper to cool at room temperature before topping. A cooling rack isn't necessary, but it does help with the overall texture. If you don't have one you could just use paper towels on a cutting board or plate.