For the dough: Combine flour, salt and sugar on the work surface. Create a well in the center and add egg and butter. Knead to form a smooth dough. Shape into a ball, wrap in plastic and chill for 1 hour.
For the filling: Meanwhile, rinse, and chop rhubarb. Boil rhubarb, sugar and vanilla in a saucepan and simmer for about 5 minutes. Remove from heat and let cool.
Preheat the oven to 200°C (approximately 400°F). Place dough on a floured surface and form a 30x20 cm (approximately 12x8 inches) rectangle. Spread filling onto dough, leaving a 1 cm (approximately 1/3 inch) border around the edges. Roll up dough, place on a baking tray lined with parchment and brush with beaten egg yolk. Bake until golden brown, about 30 minutes. Remove and let cool.
Coarsely chop chocolate and melt with cream in a saucepan. Remove from heat and let cool. Slice roulade and drizzle with chocolate sauce. Serve immediately.