10tablespoonsbutter(5 ounces), cubed and well softened
Proof yeast in hot water (about 110F). Stir to dissolve yeast, and add a pinch of sugar. Let rest 10 minutes, until yeast foams. In a stand mixer, combine flours, sugar, and salt. With the dough hook, add the eggs one by one, mixing on low/second speed. Add bourbon and proofed yeast. Mix for 10 minutes, stopping to scrape down the sides of the bowl periodically. Add butter cubes a few at a time, mixing for a minute after each addition. Mix for another 10 minutes. Scrape dough into a greased bowl and cover with plastic wrap. Let rise in a warm place for 3 hours; I put mine in my oven, preheated for 1 minute. Your dough will not double in size, but it should get a little puffy.
Turn out the dough and fold several times to redistribute the yeast. Return to the greased bowl, cover with plastic wrap, and refrigerate overnight. The next day, remove the dough from the fridge. Grease and line a 9x5 inch loaf pan with parchment paper (which will aid with removal). Shape all the dough into a loaf, or divide as I did. Take ⅔ of the dough and fold it over itself, letter-style; turn the dough and repeat if necessary until it forms a loaf roughly the size of the pan. Pat the dough into the bottom of the pan, and try to get it to the edges. Cut 2 parallel slits down the pan lengthwise. Divide the remaining dough in half, and form each piece into a rope roughly the length of the pan. Tuck the dough into the slits. Cover the loaf pan with plastic wrap and let the dough rise for another 3 hours in a warm place. The dough will not double, but hopefully it will get puffy.
Preheat the oven to 350F. Brush the top of the loaf with an egg wash spiked with bourbon, and sprinkle with sugar. Bake the loaf in the center of the oven for 35-40 minutes (mine took 43), until the loaf is golden. The temperature of the loaf should reach 190F. Cover the loaf with foil partway through baking if it is browning too much. Remove the brioche immediately from the pan and cool on a wire rack.