In a large bowl, or a stand mixer, beat butter and sugar together until combined.  

Beat in the egg yolks, increasing the mixer speed to medium and beat for 1 minute  until the eggs are well mixed

Then add in the sweet potato and half of the sour cream and vanilla extract.  Mix until the batter just comes together. 

Add in half of the flour and mix  on low speed until combined. 

Repeat alternating sour cream/flour additions.  Scrape bottom of the bowl to make sure there’s no flour left. 

Coat a large bowl with oil and put the batter in the bowl.  Cover and refrigerate for at least 1 hour, or overnight. 

Heat 2 inches of oil to 325ºF

Heavily flour the countertop and roll out dough – ½ to ¾ inches thick. Cut out donuts and place on a parchment lined baking sheet. 

For the glaze whisk powdered sugar, corn syrup, salt, vanilla extract and hot water together until smooth.  Set aside. 

Fry two to three donuts at a time.  Initially they will sink to the bottom of the pot. When the donuts float to the top, flip and fry for an additional 2-3 minutes.

Remove the donuts to rack over a baking sheet and cool for 5-10  minutes  before dipping in the glaze.