PREPARE THE YEAST: Heat the milk in a small saucepan over low heat until just warm (but not hot!) when you dip your little finger into it. Remove from the heat, then stir in the yeast. Set aside.

MAKE THE DOUGH: In the bowl of a stand mixer with the dough hook fitted (see note 1) combine 175g flour (about two-thirds), sugar, vanilla, salt and orange zest. While mixing, add the butter and egg yolk, then give the yeast and milk mixture a stir and pour it into the bowl. Run the mixer on high until the ingredients are combined, then with the mixer on low add the remaining flour. Knead for 5 minutes.

After 5 minutes the dough should be moving freely around the bowl of the mixer, and only lightly sticky. If it is still a little liquid or sticking to the sides of the bowl add one tablespoon of flour and knead for another minute.

LET THE DOUGH RISE: When the dough has finished kneading, shape it into a ball - it should feel soft, springy and elastic. Place dough back in the bowl, cover loosely with a clean cloth or some plastic wrap and allow to rise in a warm place for 1 hour.

MAKE THE CHOCOLATE FILLING: While the dough is rising, make the chocolate filling. Heat the cream in a saucepan until nearly simmering, then remove it from the heat. Place the chopped chocolate into a heatproof bowl and pour over the hot cream. Allow to stand for 5 minutes, then gently whisk until smooth. The chocolate mixture will look like it won't come together at first, but keep going! Set aside to cool. (see note 2)

ROLL OUT DOUGH: When the babka dough has risen, turn it out onto a lightly floured surface. Press out the air, then roll into a large rectangle around 1cm (⅓ in.) thick. Spread the chocolate mixture over the surface of the dough, leaving a gap around all edges. Sprinkle over the hazelnuts, and roll into a cylinder, starting from one of the short sides.

OPTIONAL: Transfer the rolled dough to the refrigerator for 20 minutes. This will make the dough MUCH easier to handle.

SHAPE DOUGH: Line a loaf pan with baking parchment paper and set aside. Use a serrated knife to cut the dough cylinder in half lengthways, leaving the very top just attached. Turn the cut sides of the dough face up, then lift one over the other to begin a rough two-stranded plait. Continue lifting the dough strands over each other, then tuck the ends under and lift into the prepared pan. Allow to rise in a warm spot for a further 20-30 minutes.

BAKE: While the dough is rising, heat the oven to 180°C / 350°F / Gas 4. When risen, bake in the middle of the preheated oven for 30-35 minutes until risen and golden brown.

GLAZE: When the babka has nearly finished baking, combine the sugar with 4 Tbsp of water in a small saucepan and heat gently until dissolved. Brush glaze over the hot baked babka, being sure to use all of the glaze. Allow to cool for 20-30 minutes before slicing and serving.