Chocolate and hazelnut pithivier

Method

1. Preheat the oven to 200°C/fan180°C/gas 6. Whizz the chocolate, hazelnuts, sugar and cocoa in a food processor until finely crumbled. Add the egg yolks, butter and rum (optional), then whizz again until the mixture forms a firm paste.
2. On a lightly floured surface lay out both pastry sheets and, with a floured knife, cut out a 24cm circle and a 26cm circle. Put the smaller one on a non-stick baking sheet and pile the chocolate filling in the centre, keeping a 1cm border at the edge.
3. Lightly beat the egg white and brush some over the border. Lay the larger circle on top, smoothing with your hands to expel any air. Firmly pinch the edges to seal. Ideally, chill for 30 minutes, loosely wrapped in baking paper, then cling film.
4. Using a knife, score (about halfway through the pastry) the top of the pithivier in curved spokes. Brush with the remaining egg white and bake for 40 minutes. (Lower the oven temperature slightly if it browns too quickly.) Serve warm or at room temperature with whipped cream.