Heat 170g of whole milk to 80f. Stir in 3 g active dry yeast and a pinch of granulated sugar. Set aside for 5- 10 minutes, until frothy and beer smelling.

Meanwhile, in the bowl of your stand mixer, add 420 g of bread flour, 50 g granulated sugar, and 10 g coarse salt. Using the paddle attachment, toss the dry ingredients together until combined.

Add in the milk and proofed yeast mixture, 130 g of active sourdough starter, 50 g of neutral cooking oil, and 1 large egg. Mix with the paddle attachment until completely combined, but avoid over mixing.

Cover dough and set aside for 30 minutes.

Install the dough hook and knead the dough for 2 - 3 minutes, or until the dough has softened and looks smooth. It will still be tacky at this stage. Recover and set aside for 45 minutes.

For the next 2 1/2 hours, every 45 minutes or so, perform a stretch and fold on the dough. To stretch and fold, gently pull the dough and fold it across itself. Then turn the bowl 90 degrees and repeat 3 more times. Recover the bowl after each stretch and fold.

During the final rest, prepare the filling. Whisk 113 g softened butter or margarine with 100 g granulated sugar and 30 g dutch process cocoa powder until smooth. Set aside.

Turn the dough onto a floured surface, and flour your hands. Press the dough into a rectangle as much as possible with your hands, then let the dough rest for a few minutes before rolling into a large rectangle about 17 x 15".

Using a bench scraper or a sharp knife, cut the dough into two equal sized pieces.

Generously spread the chocolate butter mixture over both sides of the dough. Leave a 1/2" or so of bare dough to help seal the rolls.

Sprinkle most of the grated chocolate over the butter mixture, reserving some for the top.

Once the dough pieces are generously coated with chocolate filling, roll them, lengthwise. Ensure to pinch the dough shut down the length and at each end of the rolls.

Carefully slice the rolls open, down the centre, using a serrated knife.

Lay down a piece of parchment paper and begin by weaving the 4 strands of dough.

Then moving to the right, fold each strip over the strip on its right.

With the remaining dough, moving to the left, fold each strip over the strip to the left.

Tuck any loose ends underneath the dough knot.

Use a bench knife to pick up the knotted babka and place it your desired baking dish. Trim the parchment paper to fit the dish. I used a 12" cast iron skillet.

Sprinkle the top of the sourdough babka with more grated chocolate.

If you want to bake your babka today, skip this step! Cover your baking dish and pop your sourdough chocolate babka in the fridge overnight. If you're cold fermenting, remove the babka from the fridge at least 3 hours before you want to bake, as it will need a longer time to complete the final rise.

Cover your dough, one final time, and place it somewhere warm to complete the final rise. This will take between 1 and 2 hours. The dough should nearly double in volume.

Preheat oven to 375f.

Bake sourdough babka at 375f for 25 minutes, then reduce the heat to 350f and bake for another 5-7 minutes or until golden brown.

Using the parchment paper as a sling, remove it from the cast iron skillet or baking dish immediately after baking and place on a wire mesh rack to cool before enjoying.