6 ½Tablespoonssugar,, divided, plus more for sprinkling
½cup plus 2 Tablespoonsunsalted butter,, very soft
Whisk together until combined:
pinch of Kosher salt
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Freeze fresh blueberries in a single layer. (Previously frozen blueberries are too watery.)
Warm the milk and pour into a mixing bowl, preferably that of a stand mixer with a dough hook attachment. Whisking by hand, add the yeast to combine. Continuing to whisk, add both flours, 2 ½ Tablespoons of sugar, the salt, eggs, and egg yolk to bowl. With mixer on low speed, continue to mix until dough comes together into a ball, about 1-2 minutes.
Increasing speed to medium-low, continue for 5-6 minutes, pausing to push dough down the sides and off of the hook. Continue until dough pulls away from the walls of the bowl and becomes firm bread.
Reduce speed back to low and add butter, in intervals, slowly for about 2 minutes, pushing dough off the hook as you go.
Once butter is incorporated, increase the speed to medium-high until the dough comes together, about 5-6 minutes more.
Place dough onto a lightly floured work area and press into a 16" x 10" rectangle.
With a short side nearest you; evenly add the blueberries and sprinkle 2 Tablespoons sugar along the top edge. Gently roll towards you, creating a log.
Place onto a greased cookie sheet. Wrap log and cookie sheet together in plastic.
Refrigerate for at least 1 hour, or if possible, overnight.
Oil a 9" x 5" loaf pan with vegetable oil.
Press dough to reshape into a 12" x 6" rectangle.
Sprinkle with remaining 2 Tablespoons sugar and with a short side nearest you tightly roll down toward you.
Place loaf into prepared pan and wrap loosely in plastic. Allow to rise in a warm draft-free area for about 3 hours, until almost doubled in size.
While brioche is at the end of its rising time, preheat oven to 350°F.
Brush dough with Egg Wash, being careful not to let egg pool around edges.
Sprinkle liberally with sugar.
Bake for 40-45 minutes, or until golden. Allow to cool for about 15 minutes in pan.
Transfer to a cooling rack.
Gr8 Do-Ahead Tip:
Keep wrapped at room temperature, for up to 3 days. If it lasts that long! Don't count on it!