Place milk, 4 eggs, the flour and yeast in the bowl of a stand mixer fitted with the dough hook attachment. Mix on medium speed until dough is smooth and comes together, about 3 minutes, scraping down the bowl after the first minute of mixing, if needed. Let dough rest in bowl 20 minutes to help relax the gluten.

Add 1/3 cup granulated sugar and 1 1/2 teaspoons salt; mix on low speed 3 minutes more, scraping dough off the hook after the first minute.

Add half of the 1 cup cubed butter and continue to mix on low until butter cubes are no longer visible and are completely incorporated into dough. Add remaining butter and mix until incorporated.

Once all the butter is added, increase speed to medium and mix until dough makes a slapping sound and comes away from the bowl, about 16 minutes. Be patient and resist the urge to increase the speed. You will start hearing a distinct slapping sound when dough is almost ready.

Transfer dough to a medium bowl coated with cooking spray. Cover with plastic wrap; let stand at room temperature 1 hour, then punch down dough to help redistribute the yeast. Cover again and refrigerate at least 2 hours or overnight.

While dough is chilling, prepare streusel and filling. Preheat oven to 325°. In a stand mixer fitted with the paddle attachment, combine all-purpose flour, 1/2 cup granulated sugar and 1 tablespoon cocoa powder. Add the 1/4 cup cubed butter; mix on low speed until mixture looks like loose, damp sand, about 5 minutes.

Transfer to a baking sheet lined with parchment paper; firmly pat out into a roughly 1/4-inch thickness. Bake 20 minutes. Let cool, then crumble finely.

In a saucepan, melt chocolate and 3 tablespoons butter over low heat. Fold in honey and 1 cup streusel. Refrigerate until set to a paste-like consistency, about 30 minutes.

When dough has completed its 2-hour rest in the refrigerator, lightly grease two 8x4-inch loaf pans. Divide the dough in half; roll one portion of dough into a 12x8-inch rectangle, with a short edge facing you. Evenly spread half the filling over dough. Starting from the top, tightly roll dough toward you into a cylinder. As you roll, stop twice to pull the dough spiral slightly away from you before rolling forward again. Place seam side down into pan. Cover loosely with plastic wrap. Repeat with remaining dough and filling.

Preheat oven to its lowest setting, about 170°. Place a cake pan on lowest oven rack. Add 1 cup boiling water to cake pan. Turn off oven. Place loaf pans in oven. Let sit 1 1/2 to 3 hours. When fully proofed, the centers will rise to within 1/2 inch of the tops of the pans.

Remove loaf pans from oven and remove plastic wrap. Remove cake pan with water from oven and preheat oven to 375°. Bake until deep golden brown and an instant-read thermometer inserted in center reads 200°, 35 to 40 minutes, rotating halfway through. Tent top with foil if bread is overbrowning. Carefully tip loaves out of pan and cool 20 minutes on a wire rack.

For glaze: Sift together powdered sugar, 3 tablespoons cocoa powder and the meringue powder. Add 4 teaspoons water and whisk until smooth, adding additional water to reach drizzling consistency if necessary. Drizzle over warm loaves; sprinkle with any remaining streusel.