To make dough, place flour, sugar, yeast and zest in bowl of stand mixer with the dough hook attached. Stir dry ingredients briefly to combine, then add the eggs and water and turn the mixer on to low speed. Mix until everything comes together, it will be quite crumbly but should begin to clump into a mass. Whilst the mixer is on low, add the butter gradually in approximately 8 portions, ensuring that each addition of butter is fully mixed in before adding the next. Scrape down the sides of the bowl as you go, the mixture will be quite sticky at times. When all the butter is fully incorporated, continue to mix on low speed until the dough becomes a smooth mass, this will take around 15 minutes. If the mixture is too sticky to form a smooth mass after 15 minutes, add a little more flour one tablespoon at a time. Remove dough from mixer and knead briefly on a lightly floured surface and form a smooth ball. Place in a lightly oiled bowl, cover with cling film and place in refrigerator overnight.

To make the chocolate filling, melt the chocolate and butter together in a bowl over simmering water, or in the microwave on low, stirring often until melted. Remove from heat, add sifted icing sugar and cocoa powder and whisk to combine. Set aside to cool, once cooled the mixture will form a spreadable paste. Refrigerate for a short time if required to reach a soft spreadable consistency.

Grease and line two loaf tins, around 15cm x 25cm in size. Cut dough in half and return half to the refrigerator while you assemble the first loaf. Roll dough on a lightly floured surface until it reaches approximately 25cm wide, roll in the other direction until it is approximately 35cm to 40cm long. Spread with half of the chocolate mixture, leaving a 2cm boarder at the top edge. Starting at the shorter edge, tightly roll the dough in to a log, seal the end with a little water if need be and place seam side down in the freezer for 10 minutes while you assemble the second log in the same way as the first.

Whilst the second log is in the freezer for 10 minutes, slice the first log long ways down the middle. Leave to rest for 10 minutes or so, to return to room temperature. With the cut sides facing up, place one half over the other in an angled cross and twist the two ends to form a braid, continuing to keep the cut sides face up. Pinch the ends to hold the shape, and then gently pat from both ends to form a loaf shape and place in to one of the prepared loaf tins. Repeat with the second loaf. Cover with cling film or a damp tea towel and leave to rise for a further two hours at room temperature, until nearly double in size.

Preheat oven to 190C non-fan forced (preferred), or 180C if fan forced is the only option. Bake loaves for 30 to 35 minutes or until a skewer inserted into a middle section of the dough comes out without resistance. If it looks like they loaves are browning too quickly, place some foil over the top of them for the remainder of baking time.

Whilst the loaves are in the oven, make the syrup. Combine the sugar, water and orange peel if using, in a small saucepan and bring to the boil, stirring gently. Simmer until all sugar is dissolved, remove from heat and set aside to steep and cool a little. Remove loaves from oven and immediately brush half of the syrup over the top of each loaf. Leave in pans for 15 minutes and then remove to cool on a wire rack.

Can be served warm or at room temperature. Will keep in an airtight container for several days.