Dough (20 min, then overnight): Mix flour, sugar, yeast and zest. Add eggs and water and knead 5 minutes, then the salt and the butter a bit at a time, kneading 10 minutes until smooth, shiny and pulling off the bowl. Cover and refrigerate overnight.
Filling (5 min): Melt chocolate and butter, then stir in the powdered sugar and cocoa to a spreadable paste.
Roll (10 min): Halve the cold dough. Roll each into a 38 x 28 cm (15 x 11 in) rectangle. Spread with half the filling (plus spread, if using), leaving a 2 cm border on the far side.
Roll and twist (10 min): Roll up tightly from the long near side into a log. Trim the ends. Cut the log in half lengthwise with a sharp knife, so the layers show. Lay the halves cut sides up side by side and twist them over each other, keeping the cut sides up. Lift into a buttered, lined 23 x 10 cm (9 x 4 in) loaf tin.
Proof (1 hr 30 min to 2 hr): Cover and proof somewhere warm until it has risen by a third and fills the tin.
Bake (35 to 40 min): Bake at 190C (375F) for 35 to 40 minutes until deep brown; a skewer should come out with no raw dough. Cover with foil if it browns too fast.
Syrup (5 min): Boil sugar and water to dissolve and brush all of it over the hot loaves. Cool in the tins.
Tips
Cold dough is what makes the twist possible; work it straight from the fridge.
Don't skip the syrup: it keeps the babka moist for days.