Make the dough

• In the bowl of a stand mixer, combine the flour, sugar, yeast, and salt. In another small bowl, whisk together the egg, yolk, and milk. Verify that the milk is 100–105°F (38–41°C) with your Thermapen ONE. 
• With the mixer running, stream the egg and milk mixture into the flour mixture. 
• Once a shaggy dough forms, add the butter.
• Knead the dough in the mixer until it is smooth and elastic, 5–10 minutes. 
• Remove the dough from the mixer and knead it once or twice by hand. Put it into a greased container and put it in a warm spot, about 85°F (29°C), to rise until doubled in volume, 60–90 minutes. 
• Once it has doubled in volume, chill it in the refrigerator for 45 minutes. 

Make the filling and fill the babka

• While the dough proofs, prepare the filling. In a double boiler or a bowl set over simmering water, melt together the chocolate, butter, cocoa powder, sugar, cinnamon, and salt. Stir until well combined and smooth. Remove from heat and let stand until ready to use. 
• When the dough has proofed and chilled, roll it out on a countertop until it is a rectangle about 22 x 12″.
• Spread the filling onto the rectangle going almost all the way to the edges. Roll the dough up along its longest axis to make a filled log about 22″ long. 
• Starting about 2 inches from one end, use a sharp knife to split the filled log down its length. 
• Twist the two halves of the split log together, pressing the loose ends together at the end of the twist. Bend the twist back on itself so that the two ends are touching. 
• Lift the woven dough into a buttered loaf pan, cover it, and let it proof for another 30 minutes.

Bake the babka

• Preheat your oven to 350°F (177°C).
• When the babka has proofed and the oven is hot, make the eggwash by whisking together the egg and cream with a fork. Brush it onto the loaf, then put the loaf in the oven to bake. 
• Bake the babka for about 45–55 minutes.—set your Extra Big & Loud timer for 45 minutes, and check the babka’s internal temperature when the timer sounds.
• Use your Thermapen ONE to check the temperature, looking for a temp between 180 and 190°F (82 and 88°C).
• If the babka is done, remove it from the oven to cool in its pan for 10 minutes before turning it out onto a cooling rack to cool further. 
• Slice the babka and serve!