For Nutella, soak hazelnuts in water overnight. The next day, drain and rinse well. Place hazelnuts in a food processor and whiz until fine. Add 1/4 tsp salt and pulse to combine. Add 1/4 cup (25g) cacao and whiz to combine, then add 25g coconut oil, and whiz until combined. Repeat, alternating between cacao and coconut oil, and scraping down sides of bowl, until combined.
Add maple syrup in a thin stream to form a paste. If mixture splits, add warm water, 1-2 tbs at a time, with the motor running to bring back together. Place in an airtight container and store.
For creme patissiere, combine vanilla pod and seeds, and 80ml milk in a pan over medium-low heat and bring to just below boiling point. Remove from heat and discard vanilla pod. Combine cornflour and remaining 1 tbs milk in a bowl. Add egg and sugar, and whisk until pale. Pour warm milk into egg mixture, whisking to combine.
Return to saucepan and place over medium-low heat. Cook, whisking constantly, for 3-4 minutes until thick and smooth. Transfer to a bowl and cover with plastic wrap. Chill for 20 minutes.
Preheat oven to 200°C. On a lightly floured work surface, roll out pastry into a 30cm x 30cm square. Spread evenly with Nutella, then spread with creme patissiere. Roll up the pastry into a loose swiss roll. Freeze for 15 minutes to firm up. Slice into 10 even rounds and place cut-side up on a baking paper-lined baking tray. Flatten slightly, then scatter with hazelnuts. Bake for 20-25 minutes until golden.