Heat oven to 180C/160C fan/gas 4. Roll the pastry out on a lightly floured surface and use to line a 20cm loose-bottomed tart tin. Fill the tart with baking paper and baking beans, bake for 25 mins, then remove the baking paper and beans. Continue to cook for 10 mins more until golden. Remove from the oven and cool.
Melt the chocolate and butter together in a large bowl over a pan of simmering water. Whisk the eggs and maple syrup together, then stir into the chocolate with most of the nuts. Pour into the tart shell, top with the remaining nuts and bake for 30-40 mins until set. Cool and serve with vanilla ice cream or double cream.