Mix the egg yolks, sugar, sour cream and salt with a hand mixer on medium speed.
Add the baking soda, baking powder and vanilla and continue to beat on low speed until combined.
Add 3 cups of flower and continue to beat on low speed until incorporated.
Cover bowl and put in refrigerator and chill the dough overnight (or at least 8 hours)
When you are ready to make the kringla, preheat the oven to 350 degrees F. and move a rack to the top shelf.
Line two cookie sheets with parchment paper.
On a clean, flat surface, cover your work area with a silicone baking mat if you have one and then dust that with a bit of flour. (You can also use a large cutting board, clean countertop, or pastry board as your work surface.)
Take the cookie dough out of the refrigerator and spoon out a heaping tablespoon of dough. Roll this in flour on all sides to form a ball, then cut it into 3 equal pieces with a sharp paring knife.
Roll out each section with your hands into a strip in the shape and thickness of a pencil. If the dough sticks to the surface, add a tiny bit of flour until it doesn't stick anymore. Don’t roll the strip too thin or they won’t raise. Place the strip onto the cookie sheet and form into a shape like the letter "B".
Bake at 350 degrees F. on the top rack in the oven. (This is so the bottoms won’t over-bake and get hard.)
Bake until just a very small hint of browning shows on top. (This was about 7 minutes in my oven, but watch your first batch closely to get a feel for timing as this could be as little as 5 minutes depending on your oven!)