Step 1 Place the potatoes in a saucepan with cold water to cover by at least 2 inches. Bring to a boil over high heat, then reduce the heat and simmer until the potatoes are thoroughly cooked and tender.
Step 2 Peel the warm potatoes and press them through a tamis. Immediately weigh out 9 ounces of puréed potatoes and place them in a medium metal bowl. Working quickly, whisk the flour into the warm potatoes, then whisk in 2 tablespoons crème fraîche. Add 1 egg, whisking until the batter is smooth, add the second egg, and then add the yolk.
Step 3Hold the whisk with some of the batter over the bowl. The batter should fall in a thick stream but hold its shape when it hits the batter in the bowl. If it is too thick, add a little more crème fraîche. Season to taste with salt and white pepper.
Step 4Heat an electric griddle to 350 degrees. (If you do not have a griddle, heat a large nonstick skillet over medium-low heat.) Spoon between 1 and 1 1/2 teaspoons of batter onto the griddle or skillet for each pancake. Cook until the bottoms are browned, 1 to 2 minutes, then flip them to cook the second side, about 1 minute; the blini should be evenly browned with a small ring of white around the edges. Transfer the blini to a small baking sheet and keep warm while you make the remaining blini, cleaning the skillet in between batches as needed.