Beat the eggs with sugar, salt, lemon zest and the grated tonka beans (put some of them aside for the glaze) until frothy. Sift the flour with cornstarch and baking powder twice and stir in a tablespoon at a time. Finally stir in the melted butter. Pour the dough into a large, greased loaf pan and bake for 60-80 minutes (chopsticks test!).
Melt the palm fat with the couverture over a water bath, stir with a pinch of tonka bean and cover the cooled cake with the glaze.