if using active dry yeast, combine with warm milk and allow it to bloom. it should become foamy within 5 to 10 minutes.
add the flour, sugar, and instant yeast to the bowl of a stand mixer. whisk to combine.
add the eggs, milk, and salt. if using active dry yeast, add the milk/yeast mixture to the bowl now after it has bloomed.
using the dough hook attachment of your stand mixer, knead on low speed until the dough comes together. this should take about 2 minutes.
start adding the butter, approximately 1 tbsp at a time. make sure the butter is incorporated before adding more. once all the butter is added, knead until a smooth, elastic dough forms. you may need to scrape the sides of the bowl. this will take about 10-12 minutes. the dough should not be sticking to the sides of the bowl anymore and should be stretchy.
move the dough to a large oiled bowl and cover with cling wrap. chill in the fridge overnight or at least 8 hours. alternatively, you can leave the dough at room temperature for 1-2 hours until doubled in size.
make the filling
combine butter and cream into a small saucepan. heat on medium until the butter is melted.
add chocolate and stir to melt. remove from heat before adding cocoa, cinnamon, salt, and sugar. whisk to combine.
transfer to a separate bowl and allow to thicken for an hourif the ganache splits, please see notes below for rescue instructions.
assembly
grease your loaf pan. you can also line with parchment, but this is optional.
after the dough has proofed either overnight or at room temperature, punch it down and scrape onto your floured work surface
using a rolling pin, roll the dough into a large square about ¼" thick.
spread the filling evenly onto the dough with an offset spatula
roll the dough into a tight log and pinch the seam to seal.
cut the log in half lengthwise. position the cut logs beside each other with the cut side facing up. twist the logs together (cut sides facing UP) into a braid and tuck the ends underneath. transfer to the loaf pan.
cover the pan with cling wrap or a towel and allow to proof in a warm place for 1.5 to 2 hours until puffy. it should be close to double in size.
preheat over to 350℉
bake the loaf for 40 minutes. the tops should be golden. if you notice the tops are getting too dark, cover with aluminum foil to prevent further browning. the loaf is done when the internal temperature reads between 190℉ and 200℉.
for the simple syrup
combine sugar and water in a small saucepan and heat until the sugar is completely dissolved. alternatively, you can microwave the mixture, stirring occasionally until the sugar dissolves.
finishing touches
once the babka is cooked through, remove from the oven and immediately brush with the simple syrup.
allow the loaf to cool in the pan for 30 minutes before removing.
cut into thick slices and serve warm.
Notes
if you are using dry active yeast and it is not blooming after 10 minutes, stir one tsp of sugar to the yeast mixture and place in a warm place. to create a warm environment, microwave a cup of water until steaming, remove from microwave and place the yeast mixture inside of the warmed microwave. DO NOT microwave the yeast mixture as this will kill the yeast.
the purpose of brushing the loaf with simple syrup is to seal in the moisture. do not skip this step, otherwise you will end up with a dry babka.
to freeze - cool loaf completely. slice entire loaf and wrap each slice in cling wrap. bundle 3 cling wrapped slices together and wrap in aluminum foil. place the bundles in a freezer bag or air tight container for up to 2 months.
to reheat, remove all wrapping and microwave for 30 seconds.
saving a split ganache
heat two tablespoon of heavy cream in a separate saucepan until it is steaming slightly. you do not want to overheat the cream because this is what caused the ganache to split in the first place!
remove the cream from heat
add a small amount of the split chocolate filling to the heated cream and whisk quickly to incorporate. repeat until all or most of the chocolate solids have been added. you may need to leave some of the separated fat out. let it go, it will be okay.
whisk until you achieve a smooth, shiny texture. proceed with the recipe as normal!