To make the dough begin by combining flour, sugar and yeast in the bottom of the bowl of a stand mixer fitted with the dough hook attachment. Mix to combine. Add egg, egg yolk, Rodelle Gourmet Vanilla Extract and ¼ cup water, mixing on low speed until the dough comes together. This will take a few minutes but you can add water a ½ tablespoon at a time to help the dough come together if it is too dry. With the mixer still on low, add the salt and then the butter, one tablespoon at a time, letting each addition fully incorporate into the dough before adding the next. Increase the speed to medium and mix the dough for 10 minutes until the dough is smooth and elastic. Stop the bowl occasionally to scrape down the sides. If the dough is not pulling away from the sides of the bowl after 10 minutes, add one tablespoon of flour to help bring the dough together.
Spray a large bowl with nonstick cooking spray or canola oil and transfer the dough from the mixer bowl to the coated bowl. Cover with plastic wrap and refrigerate overnight. Dough will expand slightly but will not double in size.
Before removing dough from refrigerator, make the filling and allow it to cool to room temperature. To make filling, combine butter and chocolate in a medium saucepan over medium heat. Stir often until melted and smooth. Add cocoa powder, confectioner’s sugar and Rodelle Gourmet Vanilla Extract. Stir until smooth and no clumps remain. Set aside to cool slightly.
Spray a 9 x 4 inch loaf pan with non-stick cooking spray and line the bottom with parchment paper, allowing the ends to hang over the edge of the pan for easy removal. Remove dough from refrigerator. Roll the dough on a well-floured surface until it forms an 11 x 13 rectangle, placing the short side nearest you. Use a pastry or pizza cutter to trim the outside edges so it forms a nice even rectangle.
Place dollops of the cooled chocolate mixture all over the dough and use and offset spatula to spread the chocolate evenly. Leave a ½ inch border around the outside of the dough. Sprinkle chopped hazelnuts evenly over chocolate mixture. Dampen the end farthest from you with a bit of water. Roll the dough up starting with the short side nearest you, being careful not to press too hard on the dough. Seal the dough together at the seam and rest it seam side down on parchment lined cutting board. Place in freezer for 15 minutes.
Remove dough log from freezer and use a sharp knife to cut ½ inch off each end of the dough. Next, cut a straight line down the center of the dough, lengthwise. Lay the two pieces next to each other, cut sides up. Take the two pieces and form an X, with one piece laying over the center of the other. Working from above the center, twist the two pieces together two times and seal ends together gently. Then move to the other side of the X and twist that end together two times, sealing the ends. Try to keep the cut sides facing up.
Carefully transfer the dough to the prepared pan and cover with a tea towel. Let stand at room temperature for 2 hours to rise.
About 30 minutes before dough is ready to bake, preheat oven to 350 degrees. Whisk together one egg and one tablespoon of water to make the egg wash.
When dough is ready to bake, brush egg wash on top of the loaf with a pastry brush. Place loaf on center rack of oven and bake for 30 minutes. Check dough after 20 minutes to be sure it’s not browning too much on top. Lightly cover loaf with aluminum foil if needed. When the babka is done, the internal temperature should read 190 degrees and a toothpick inserted in the center should go in with no resistance.
While babka is baking, make sugar syrup by combining brown sugar and water in a small saucepan over medium high heat. Reduce heat to a simmer and whisk occasionally until the sugar dissolves completely. Remove from heat and set aside to cool slightly.
Remove babka from oven and brush the sugar syrup over the top immediately while loaf is hot. Allow babka to cool in the pan for 30 minutes before removing from pan and cooling completely on a wire rack.
**Babka is best served the day it is made. May be stored tightly covered in a cool, dry place. Reheat leftovers before serving.
