This sweet bread is similar to buttery brioche, but has crumbled sugar cubes baked into the dough for a tasty crunch when you take a bite. For a thick, opaque glaze, add additional confectioners' sugar until you reach the desired consistency.
Dough: Sprinkle yeast over warm milk in a bowl; stir until dissolved. Let stand until foamy, about 5 minutes.
Place flour, 1 tablespoon salt, superfine sugar, 6 eggs, and zest in the bowl of a mixer fitted with the dough-hook attachment. Add yeast mixture; mix on low speed until combined, about 5 minutes. Scrape down sides of bowl; mix on medium speed until dough is smooth and elastic, about 8 minutes.
Reduce speed to low; add butter, a few pieces at a time, and knead with dough hook until incorporated. Increase speed to medium; continue kneading until dough is smooth and shiny and pulls away from side of bowl, 6 to 8 minutes. Cover with plastic wrap; let stand in a warm spot until doubled in bulk, 1 1/2 to 2 hours. Refrigerate at least 8 hours and up to 12 hours.
Butter two 8-by-41/2-inch loaf pans. Place sugar cubes in a large resealable plastic bag and crush into pea-size pieces with a rolling pin.
Turn out dough onto a lightly floured surface and divide into two equal pieces, then divide each piece into eighths. Pat each piece into a 3-inch square about 1/2 inch thick; place about 2 teaspoons sugar pieces in center of each. Gather corners up over sugar to enclose; pinch to seal. Flip over and roll into a ball. Place 8 balls in each pan, in 2 rows of 4.
Cover each pan loosely with plastic wrap. Let rise in a warm spot until doubled in bulk, 1 1/2 to 2 hours.
Preheat oven to 425 degrees. In a bowl, whisk together remaining egg and pinch of salt. Brush egg wash over each loaf. Bake until beginning to turn golden, about 15 minutes. Reduce heat to 375 degrees and continue baking until deep golden brown and a thermometer inserted in middle registers 180 degrees, 15 to 20 minutes. (If browning too quickly, tent with foil.) Let cool in pans on a wire rack 5 minutes. Turn out onto rack; let cool completely, top-side up.
Glaze: In a bowl, whisk together confectioners' sugar and milk until smooth. Drizzle over loaves. Unglazed loaves can be stored, wrapped in plastic, up to 1 day.