Grease a 20cm cake tin and cover base with a disk of baking paper.
Place flour in a processor with the 1 tbsp of almonds, salt and cubed margarine. Pulse until you have breadcrumbs.
Mix the sugar and eggs together, add to the flour and pulse until a dough forms. Roll into a ball, with the help of some flour then cover in plastic wrap and chill 1 hour.
Preheat oven to 170°C.
Mix almonds, sugar, flour, polenta flour, zest, juice, essence and eggs. mix with your hands until well blended.
Roll out the pastry and line the bottom and sides of the dish. Pour the mixture in the pastry case and bake for about 75 minutes (but start checking after 60 minutes) or a bit longer.
The cake should feel firm and springy and a skewer should come out clean.
Cool for an hour before turning out onto a rack. Cover when cool and leave for at least 1 day before covering with rolling icing.
This cake should be cut in cubes rather than slices.
