Instructions

1. For the Dough:
2. Using a stand mixer with the dough hook attached stir in the yeast with one tablespoon warm water and 1 TBSP brown sugar just until sugar and yeast have dissolved. Let sit for 5 minutes.
3. Combine the flours, oatmeal, salt and 1/3 cup brown sugar and mix on medium speed.
4. Reduce mixer speed to low and add the milk, eggs and vanilla. Beat until dough is shiny and elastic, 10 to 15 minutes.
5. Add 8 TBSP unsalted butter one TBSP at a time until all the butter is fully incorporated, then beat on low speed for about 5 minutes. Dough will not form a ball, it will be a sticky sort of blob. That's what it's supposed to do.
6. Transfer the dough to a large bowl, cover with plastic wrap and set aside. When dough has doubled in size, after 2 hours, flour a cutting board and your hands. Then push dough down on board, cover well with plastic wrap and refrigerate for 1 to 2 hours or overnight. I prefer overnight but everyone has their preference.
7. For the streusel:
8. Whisk together 11/2 cups flour, granulated sugar, brown sugar, and 1/4 tsp. salt in a bowl. 
9. Cut butter into flour mixture using a pastry blender until butter is pea-sized. 
10. Mix oats into flour mixture with your hands until thoroughly combined. 
11. Cover and refrigerate while you prepare the filling.
12. For the filling:
13. Toss together apples, brown sugar, lemon juice, flour, cinnamon, and salt in a large bowl. 
14. In a medium saucepan add 2 TBSP unsalted butter until it melts over medium heat.
15. Add the apple mixture and add 1/2 cup apple cider/juice.
16. Cook down the mixture until apples can be mashed. If the apples are not getting soft, add more cider/juice if it evaporated before the apples got soft enough. Mash apple mixture with a fork and let cool.
17. Line two 8 1/2 by 4 1/2-inch loaf pans with parchment paper, not letting paper come more than 1 inch above top of pans.
18. Remove dough from refrigerator and divide it in half. On a lightly floured surface, roll one piece of dough into a 12 x 8-inch rectangle.
19. Using a spatula spread 1/2 the apple filling over dough within 1/2 inch of edges.
20. Beginning with long side, roll dough up like a cinnamon roll and cut it straight down the middle so the filling is exposed.
21. Cut ½ inch off each end.
22. Layer each cut piece on top of one another and twist.
23. Filling will come out, that is to be expected.
24. Place in a greased loaf pan.
25. Melt remaining 2 tablespoons butter. Brush babka top with melted butter and sprinkle with half the streusel.
26. Repeat with second half of babka dough. Cover loosely with plastic wrap. Let loaves rise at room temperature until they reach top of pan or about doubled in volume, about 2 hours.
27. Heat oven to 350 degrees and bake for about 45 minutes, or until a cake tester inserted in center comes out clean. Allow babka to cool for 30 minutes before icing.
28. For the icing:
29. Whisk all the ingredients together and drizzle over the two babkas.