Mix 2 1/2 cups of flour, the salt, and 3/4 cup of sugar in a mixing bowl, add half the butter, mix well, and as Sarah’s grandmother did, make a well in the center. Mix the yeast with 1/4 cup of warm water and stir in. Add the 2 whole eggs and the sour cream, mixing well with a spoon. (You can also put everything in a standing mixer.) Then cover with plastic wrap and let sit for about 2 hours until the dough doubles in volume.
Using your fingers, blend the remaining 1/2 cup of flour and 1/4 cup of sugar with the remaining butter and set aside in a bowl.
Preheat the oven to 350 degrees and cover two baking sheets with parchment paper.
Dust a baking board with flour and roll out the dough to a circle about 1/8 inch thick, adding more flour if the dough sticks. Cut into 12 three-inch rounds and press at least 12 blueberries into each round of dough and sprinkle with a little of the remaining tablespoons of sugar and the candied ginger if using. Then, cradling the filled circle of dough in one hand, use your second hand to carefully pinch it closed into a 3-by-5-inch oblong shape. Repeat with the remaining dough and blueberries. Put the buns on the baking sheets.
Brush the buns with the remaining egg yolk, pat the streusel – the flour-sugar-butter mixture – on top of the buns, and bake for about 25 minutes or until golden – don’t worry if some of the juices boil over. That is part of the buns’ charm. Serve warm or at room temperature.