In a stand mixer, combine all of the dough ingredients: flour, sugar, salt, yeast, eggs, warm water or milk, and butter. Using the dough hook, mix until the ingredients come together into a mostly smooth, shiny dough. You might notice that it starts as a sticky mess, but don't worry; it will become beautifully satiny after kneading. Knead for about 5 minutes on medium speed.
Spray a medium bowl with cooking spray. Transfer the dough into the greased bowl using a spatula, as it will be a bit sticky. Cover the bowl with a clean kitchen towel and let it rise for about 1 hour until it doubles in size. If you're planning ahead, you can cover it with plastic wrap and refrigerate it overnight.
In a clean bowl of the stand mixer, add the filling ingredients: butter, cocoa powder, brown sugar, white sugar, honey, cinnamon, allspice, salt, and cloves. Beat until well combined. You can mix this by hand with a fork if you prefer.
Grease and line either an 8×4-inch or 9×5-inch loaf pan. On a liberally floured surface, dump the dough out and roll it into a rectangle, about 10 inches x 12 inches. Don’t hesitate to use a measuring tape for accuracy.
Spread the filling evenly over the dough, leaving about 1/4 inch from the edge. Roll the rectangle tightly, horizontally, into a log. Transfer the log to a lightly floured cutting board and place it in the freezer for about 10 minutes; this will help when cutting and shaping the babka.
After the log has chilled, remove it from the freezer. Use a serrated knife to slice off the ends and discard them. Slice the log in half and lay the two halves next to each other, cut side up. Pinch the tops together to seal them, and then lift one strand over the other to form a twist. Try to keep the cut sides up as you work your way down the log.
Carefully transfer the twisted loaf to the prepared pan. Cover with a kitchen towel and place in a draft-free area to rise again for about 1 hour.
Preheat the oven to 350 degrees F. Brush the tops of the babka with the whisked egg yolk for a beautiful golden crust. Bake for 30 to 35 minutes, or until the tops are lightly golden brown and the loaf sounds hollow when tapped.
While the babka is baking, prepare the rhubarb glaze. In a baking dish, combine the pieces of rhubarb, sugar, and citrus juice. Roast in the oven for about 10 minutes until very softened. Remove the rhubarb and transfer it to a blender. Pulse until smooth, scraping down the sides as necessary.
In a bowl of a stand mixer, sift in the powdered sugar and a pinch of salt to the rhubarb puree. Beat until smooth. Adjust the consistency with more lemon or lime juice if it’s too stiff or add more powdered sugar if it’s too thin.
Once the babka has cooled slightly, remove it from the pan and drizzle the rhubarb glaze over it in your desired pattern. You can use a piping bag with the end snipped off for even stripes. Enjoy your delicious creation!
