Place the butter in a bowl, whisk in the sugar and salt flakes using the whisk attachment on a hand mixer. Whisk in the egg, lemon zest and vanilla seeds. Combine the flour, almonds and baking powder, add to the mixture, mix quickly to create a soft dough – do not knead. Flatten the dough, cover and chill for approx. 30 mins.
Roll out two thirds of the dough between two sheets of baking paper to a thickness of approx. 3 mm, place in the tin, press the dough firmly around the edge of the tin. Prick the tart base firmly with a fork. On a lightly floured surface, roll out the remainder of the dough into a circle (approx. 4 mm thick) to match the size of the tin, place on a sheet of baking paper. Chill the base and lid for approx. 30 mins.
To roast the nuts
Spread the walnuts and pine nuts on a baking tray lined with baking paper. Roast for approx. 10 mins. in the centre of an oven preheated to 180°C. Remove the nuts.
Warm the honey in a pan, gradually add the sugar. Reduce the heat, swirling the pan occasionally until a brown caramel has formed, pour in the cream, reduce for approx. 5 mins. until the caramel has dissolved. Add the nuts, salt flakes and allspice, mix, leave to stand for approx. 2 mins. Spread the warm nut filling over the tart base.
To cover the tart
Place the lid on top of the tart, press the edges down firmly with a fork, prick.
Approx. 20 mins. in an oven preheated to 220 °C (convection). Remove from the oven, sprinkle the salt flakes on top, allow to cool slightly, remove the tin frame, slide the tart onto a cooling rack and leave to cool.