1) If using the anise seeds, lightly toast them in frying pan over medium heat until aromatic, 2 minutes. Allow to cool and grind in a spice mill or mortar and pestle until fine.
2) Combine the butter and sugar in a food processor until creamy. Add the egg, anise seed if using, vanilla and salt and mix to combine. (This may look rough and curdy, but don't worry, it all comes back together.) Add about half of the flour and mix on high speed 3 to 4 times to incorporate. Scrape down the edges of the processor and add the remaining flour. Mix just enough to make a coarse, crumbly dough.
3) Remove from the mixing bowl and press dough into the bottom and up the sides of a 23-cm springform pan to form a thin, even layer, using greaseproof paper to smooth. Freeze the pastry whilst you prepare the filling.
For the filling:
1) Preheat oven to 180C/Gas Mark 4.
2) Chop 175g almonds in a food processor until very finely ground (take care not to overprocess the nuts or they can get oily) and transfer to a bowl. In the same processor, combine the butter and sugar until smooth. Add the cinnamon and continue to mix until evenly combined. Scrape down the sides of the bowl and beat in the egg. Add the almonds and mix until just blended.
3) Arrange the plums cut side down, over the pastry. Spread the filling on top and smooth with a spatula or the back of a spoon. Sprinkle with the remaining almonds and place in the freezer for 30 minutes.
4) Remove the pie from the freezer, place in the oven and bake for about 1 1/4 - 1 1/2 hours, until the filling puffs up and is golden but still slightly soft in the centre. If the crust begins to overbrown, cover with foil during the last 15 minutes of cooking. When pie is ready, turn off the oven and open the door, leave the pie for 15 minutes and then transfer to a rack to allow it to cool completely.
5) Serve at room temperature with ice cream or lightly sweetened whipped cream.
1) The pie can be assembled and frozen, before baking, up to 2 weeks in advance. Bake the frozen pie for about 1 1/2 hours.
2) The baked pie can be tightly wrapped and refrigerated for up to 2 days, or frozen for up to 2 weeks. Allow to reach room temperature before serving.