In the bowl of a stand mixer, whisk together the yeast, sugar, salt and 125g of the flour together.
In a microwave or in a small saucepan over medium-low heat, warm 60g water, milk butter and vanilla. Add the warm mixture to the bowl in the stand mixer and beat (using a paddle attachment) on low until just combined. Add 1 egg and mix on low. Add remaining flour (185) and beat on low, until the mixture is just combined and you no longer see streaks of flour
Switch to your dough hook attachment and beat on a medium-low speed for 5-7 minutes, until the dough starts to pull away from the sides of the bowl but the dough itself is still sticky when touched. If it isn’t pulling away, you can add up to 1 extra tablespoon of flour
Scrape dough out onto a lightly floured surface and do a series of stretch and folds. Grab the top portion of the dough and pull it up, then bringing it on to the middle of the dough ball. Do a quarter turn, and repeat until 4 stretch and folds have been done. This isn’t a ‘must do’ BUT will give you a fluffier babka
Oil your stand mixer bowl, place dough inside and cover with plastic. Let it rise in a warm, draft-free place for 45 minutes, or until doubled in size
Scrape the dough back on to a lightly floured surface and press it out with your fingers, or with a rolling pin, until it is 8x6 inches. Wrap it with plastic and refrigerate it for at least one hour, or overnight. While one hour is fine, the flavour will improve if you refrigerate it overnight
While that is rising, make the filling. Melt butter and molasses together until smooth. Stir in the sugars and spices. Let cool to room temperature.
Grease a loaf pan with butter or baking spray and line with baking parchment.
On a lightly floured surface, roll out chilled dough until it is 13x10 inches. Using a pastry brush or small offset spatula, brush the filling over the dough but leave a ½ inch border all around the rectangle. Sprinkle with the chopped chocolate.
Starting with one long side, tightly roll it up. Carefully lift it up and pull each end until the cylinder is 14 inches. With the seam of the cylinder facing down, use a bread knife to cut the cylinder in half. Overlap one cylinder over the other, with the filling side up, forming an X. Twist the upper half twice, then the lower half twice, and pinch the ends. Carefully place in the prepared loaf pans, tucking the ends under
Cover and let rise in a warm place for 30 minutes - 1 hour (depending on how long you chilled it for), then test if it is ready to be baked with the finger dent test. Gently press your finger into the surface. You should be able to watch the dough spring back slightly but still show an indentation.
Preheat oven to 350°F/180°C. Whisk together the remaining egg and water, then use a pastry brush to brush the egg wash on top of the dough.
Bake for 45-50 minutes, until golden brown. While it is baking, make your simple syrup by heating up the sugar and water in a microwave or in a small saucepan over medium heat until the sugar has completely dissolved. Let it cool before using.
Once out of the oven, brush with the simple syrup. Let cool for at least an hour before cutting.