For the pie crust:
1) Combine the flour, ground pecans, sugar and salt in a large bowl. Add the butter and rub the butter pieces between your fingertips and thumbs, or use a pastry blender, until the mixture resembles coarse crumbs.
2) Pour in the ice water and work it in until the dough holds together, being careful not to overmix. Squeeze a small amount together; if it is too crumbly, add some more ice water, a little at a time. Shape it into a smooth ball, flatten into a disk, and wrap in cling film. Refrigerate for at least 30 minutes, and up to 2 days.
For the pie:
1) Preheat the oven to 190C/Gas 5.
2) Roll out the dough into a circle, about 30cm in diameter. Gently press it into a 20-cm pie tin and crimp edges with your forefinger and thumb. Prick the bottom of the pie crust all over with the tines of a fork to keep it from puffing up in the oven. Refrigerate until well chilled, about 45 minutes.
3) Put the sweet potatoes on a baking tray and bake in the oven until very tender, about 45 minutes. Turn the oven down to 180C/Gas 4 and remove potatoes from oven; allow to cool slightly before scooping the flesh into a food processor. Puree until smooth, then add the eggs, cream, brown sugar, sugar syrup and vanilla extract. Pulse a couple of times to combine, then add the cinnamon, nutmeg, and salt.
4) Put the pie pan on a rimmed baking tray and pour in the filling. Bake until the center is set, about 40 minutes. Let cool completely, then transfer the pie to the refrigerator.
5) To make the praline topping, melt the butter in a small saucepan over medium-low heat and add the sugar, syrup, pecans and salt. Cook until the sugar dissolves and the mixture is smooth, about 3 to 5 minutes. Remove from the stovetop and let cool for 5 minutes.
6) Remove the pie from the refrigerator and pour the praline topping over the cooled pie, spreading evenly with a spatula or by gently rolling the pie around at an angle so it hits the edges. Cool until topping has hardened, about 20 minutes. Serve.