In a medium saucepan heat butter and 2/3 cup of milk together just until the butter is melted. Remove from heat and stir in vanilla. Set aside to cool to 115°F to 125°F.
In a small bowl whisk 2 tablespoons sugar with the warm water until dissolved. Whisk in the yeast and set aside until foamy, 5 to 10 minutes.
In a large bowl whisk the remaining 6 tablespoons of sugar with 4 cups of flour and salt until combined. Add the milk mixture, yeast mixture, and eggs. Stir with a wooden spoon until a sticky dough forms.
Add another 1 1/2 cups of flour and continue to stir until smooth. Dough will still be sticky and that's o.k. Shape dough into a loose ball and cover with plastic wrap. Place in a warm place and let dough rise until doubled, 1 to 1-1/2 hours.
In a small bowl whisk sugar and cinnamon for the filling together.
Punch down risen dough. Add remaining 1/4 cup of flour and knead into dough. Let rest 5 minutes, covered with a clean kitchen towel.
Divide dough into 2 equal-sized pieces. Lightly dust working surface with all-purpose flour. Roll each piece of dough into a 20 by 12-inch rectangle with a floured rolling pin.
Brush rectangle with melted butter and then sprinkle with half of the sugar-cinnamon mixture. Use a pizza cutter or sharp knife and cut dough into 6 pieces down the 12-inch length. Stack the strips on top of each other and then cut stack into 6 even stacks of squares.
Spray two 9x5 loaf pans with baking spray. Line with parchment paper. I use two strips and criss-cross them. Spray the parchment paper. Prop up the pans vertically and carefully stack a few squares into the pan at a time, tucking slices of apple evenly between the two pans. Unprop the pan and cover loosely with a clean kitchen towel. Complete the same method in the second pan and cover with a clean towel and let rise 30 to 45 minutes until doubled.
Preheat oven to 350°F. In a medium-sized bowl combine the streusel ingredients except for the butter. Cut the butter in using your fingers, pizza cutter or two forks until large crumbs develop. Refrigerate until you're ready to bake.
Uncover risen loaves. Sprinkle with the streusel topping. Bake loaves for 40 minutes or until the top of the loaves are a deep golden brown.
Removed baked loaves and let cool on a baking rack in the pans for 30 minutes. Carefully lift the loaves from the pans, using the parchment paper. Drizzle with glaze.
Store baked loaves in an air-tight container or wrapped in plastic wrap and then foil at room temperature up to 3 days.
Recipe provided by http://redstaryeast.com/apple-streusel-pull-apart-bread/