1. Make the flour roux by combining the water and flour in a small saucepan over low heat. Whisk constantly until the mixture thickens to the consistency of bechamel sauce. Set aside to cool.

2. Combine the milk, castor sugar, and fresh yeast in a small bowl and set aside until bubbles start to form on the surface (this means the yeast is activated). Set aside.

3. Add the dry ingredients to the bowl of a stand mixer fitted with the dough hook. Add the flour roux mixture to the bowl and mix on low until just incorporated, then slowly add the yeast mixture. Add the eggs, one at a time, increasing the speed to medium until a dough starts to come together.

4. Add the vanilla bean paste then add the butter, a few knobs at a time, and mix until the dough is smooth and shiny. It should be quite soft, tacky and malleable. Cover the bowl with plastic wrap and rest in a warm place for about 2 hours or until the dough has doubled in size.

5. Add the dark chocolate and butter to a small saucepan. Place over low heat and whisk constantly until the chocolate starts to melt. Remove pan from heat and allow chocolate to melt completely in the residual heat.

6. Grease and line a high-sided, loose-bottomed 20-centimetre round cake tin. If your dough still seems tacky, spread a generous layer of flour onto your work surface before you begin.

7. Roll the dough out into a large rectangle about 23-centimetres wide and 40-centimetres long.

8. Spread the chocolate filling over the dough. Roll the dough up like a Swiss roll, starting from the short edge.

9. To shape the babka, hold each end of the log and twist gently in opposite directions. Fold the twisted log in half. Grab the ends and twist in opposite directions again. Repeat once more until you see some of the lines of chocolate come through – the more twisted and uneven the better.

10. Bring the ends together and place into cake tin. Cover with plastic wrap and leave to prove for 40 minutes or until the dough has risen almost to the edge of the tin.

11. Preheat oven to 180C. Brush the dough with the egg yolk and push half the chocolate chunks into the top of the dough. Bake for about 35 to 45 minutes – it should rise a little further and brown on top, and have some resistance to the touch. (Optional: after 30 minutes push the remaining chocolate chunks into the loaf to melt on top.)

12. Remove from oven and allow to cool for 10 minutes. Slice and serve while still warm.

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