Spread hazelnuts on a lightly oiled tray. Combine sugar and 2 tbsp water in a small saucepan and stir over medium-high heat until sugar dissolves, bring to the boil and cook for 5-6 minutes or until dark golden in colour, then pour over nuts and stand for 10 minutes or until caramel hardens. Break into small pieces and process in a food processor until finely ground. Store praline in an airtight container until required.
Combine chocolate and cream in a heatproof bowl placed over a saucepan of simmering water, stirring occasionally, until melted and smooth. Add praline, stir to combine, then remove from heat and refrigerate for 2 hours or until just firm.
Preheat oven to 180C. Halve puff pastry and roll each piece to 4mm thick, then place on baking paper-lined trays and refrigerate for 30 minutes. Cut a 20cm-diameter round from each piece of pastry, then place a round on a baking paper-lined oven tray. Spread chocolate mixture over pastry, leaving a 2cm border, brush edges with eggwash, then place remaining
round of pastry over chocolate mixture, pressing to seal. Using a fork, press firmly around edges, score top and brush with eggwash. Bake for 10-15 minutes or until dark golden in colour, then remove from oven and cool on a wire rack for 30 minutes. Serve warm or at room temperature, with cream passed separately.