In a small saucepan heat buttermilk, sugar, butter, and salt together just until the butter melts. Remove and allow to cool to room temperature.
Meanwhile, in a small bowl add yeast and warm water together. Mix with a fork and allow it to sit for 5 minutes or until bubbly.
Using a stand mixer, fit a dough hook and secure bowl. Add the buttermilk mixture and 1 cup of flour to the bowl and mix on medium, once combined mix in your yeast.
With the mixer on medium speed add in a 1/2 cup of flour at a time until the dough comes together. Scrape down the sides as needed. When the dough is just slightly sticky, turn mixer to high speed. and allow to mix for 5 minutes. If the dough has not come together and is too sticky through the mixing time, turn mixer off and add 1/4 of flour as needed.
When dough reaches its elastic consistency, remove from mixer and place in a buttered bowl. Cover with a dish towel and allow to rise in a warm spot until doubled. (This may take anywhere from 30–90 minutes depending on the temperature of your house and the brand of yeast).
Once dough has risen. Remove from bowl on a lightly floured surface. Roll dough out until it is about 1/2 inch thick. Using a round cookie or biscuit cutter cut 3-inch circles of dough out. Set aside on a parchment paper lined cookie sheet, cover with a dish towel to rise until doubled in size. With the dough scraps you can roll them into balls or cut them into small pieces. Also set aside to rise.
Heat your fryer to 375º F.
After the doughnuts have risen, give each a small press flattening them just slightly, place them in the fryer two at a time. Fry until golden all over. Once finished, place them on paper towels to drain the extra grease.
Poke a hole on the side of the donut using a large skewer (or the tip of a wooden spoon). Use the skewer or spoon to hollow out a cavity.
While doughnuts are still hot roll in sugar. Set aside to cool completely.
To make your filling, add the Original Goat Cheese, sugar and blueberries together in a food processor. Pulse until combined. Whisk one egg into the mixture.
In a double boiler, add mixture and whisk constantly until thickened. Once thick, transfer to a bowl and place in the refrigerator until cold.
Once cool add to a piping bag with a medium to large tip or use a plastic storage bag with a tip cut off. Gently squeeze about 1 heaping tablespoon filling into each donut.
Store doughnuts in an airtight container in the refrigerator.