1. Prepare dough. Cut butter into cubes. Set butter and eggs out to come to room temperature. Place milk in microwave-safe cup; microwave on HIGH 1 minute or until warm (about 100°F).
2. Place yeast, milk, and sugar in bowl of stand mixer; mix to blend. Let stand 10 minutes or until foamy. Place dough hook on mixer; mix in butter, eggs, and salt on LOW.
3. Add flour gradually until fully incorporated. Increase mixer to MEDIUM; knead 5–6 minutes until dough is smooth. (Add more flour 1 tablespoon at a time if dough is sticky.) Cover bowl with plastic wrap and let stand to proof 30 minutes.
4. Meanwhile, prepare filling. Place butter and chocolate in microwave-safe bowl. Microwave on HIGH 1–2 minutes, stirring every 30 seconds, until combined and smooth. Stir in cocoa and cinnamon.
5. Dust work surface with flour. Roll out dough to 20- x 12-inch rectangle. Spread filling over dough, leaving ½-inch border. Roll up tightly starting from 1 long edge; pinch to seal.
6. Coat 10- x 5- x 3-inch loaf pan with spray. Cut dough log in half lengthwise, place halves cut-side up, and twist together, keeping cut sides facing up. Pinch ends together to seal, then tuck under loaf. Place loaf in pan, cover loosely with plastic wrap, and let stand to proof 30 minutes or until doubled in size.
7. Preheat oven to 350°F. Place pan on middle rack; bake 45–55 minutes until internal temperature is 190°F and skewer inserted in center comes out mostly clean.
8. Prepare syrup. Combine water and sugar in small saucepan and bring to a boil. Boil 4 minutes, stirring occasionally. Remove from heat and let stand to cool.
9. Brush (or drizzle) babka with syrup. Let babka stand to cool completely before removing from pan. Slice and serve.

Chef's Tip: If top of loaf is browning too quickly, tent loosely with foil for last few minutes of baking.