Using a food processor or by hand, mix the dough ingredients until combined. Wrap in plastic wrap and refrigerate for at least 4 hours.
In a saucepan, heat the cream and butter over medium heat until they come to the boil. Add the pieces of chocolate. Take off the heat and let it melt for 1 minute. Stir with a spatula until smooth and even.
Shaping the Dough
Preheat the oven to 170°C (325°F/Gas 3). On a floured work surface, roll out the dough with a rolling pin to make a round about 30 cm (12 inches) in diameter. Spread the ganache with a spatula over the whole round. Mix together the topping ingredients and scatter over the ganache, setting some aside for sprinkling on top. Cut the round into 12 equal wedges, like a pie, and roll each one up, starting with the wider end, to make a small crescent. Place the completed crescents, as you make them, on a baking tray lined with baking paper.
Brush the rugelachs with the mixture of egg yolk and water. Sprinkle over the reserved chocolate–sugar topping.
Bake in the oven for about 20 minutes until the crescents are golden brown. Allow to cool for at least 10 minutes before serving.