In the bowl of the stand mixer, set on the hook attachment, and place the yeast, flour, salt, and sugar. Mix well.
1 teaspoon Dry yeast, 1 cups All-purpose flour, ⅓ cup Granulated sugar, 1 teaspoon Salt
Make a well in the dry ingredients and add the milk and eggs. Mix well. Keep the mixer on low and let the dough mix for a good 5-7 minutes. After this time, touch the dough, and if it is too sticky, add one tablespoon of flour. Mix for another 2 minutes.
¼ cup Milk, 6 Eggs
Once the dough is not sticky anymore, start adding the butter, one tablespoon at a time, and wait for the butter to mix into the dough completely before adding the next piece.
1 cup Butter
At this point, the dough should be pulling from the sides and not sticking anymore.
Transfer the dough onto a lightly floured surface, and make the dough into a ball shape. Place this large ball into a bowl and cover it with cling wrap. Place the bowl in the fridge for 3-5 hours.
Lightly flour a surface and roll out the dough into a large disk with ½ inch thickness. Cut the dough with the round cookie cutter. Place them on a lined baking sheet and cover them with cling wrap. Let them sit at room temperature for one hour.
Pour the oil up to ⅓ of the pot, and heat it to reach 350°. Use a candy thermometer for accuracy.
Place the cut dough into the hot oil until both sides are golden brown. Remove from the oil and place on a cooling rack to cool down.
If using sugar coating, dip one side of each donut into the sugar when they are HOT.
If filling them up with jam or custard, let them cool down then fill them up with a piping bag.