Grease a springform tin and preheat oven to 190 C / GAs 5.
In a bowl beat egg whites with sugar until stiff.
In a separate bowl whisk together butter, melted chocolate, icing sugar und egg yolks with a handmixer, until foamy. Add lemon zest, ground almonds and rum and stir well.
In another bowl mix flour with corn starch and baking powder and stir into the egg yolk mixture. Carefully fold in the stiff egg whites.
Transfer mixture to the prepared springform tin and bake in the preheated oven until a wooden skewer inserted in the center comes out clean, about 45 to 60 minutes. After 30 minutes, cover the cake with greaseproof paper to prevent it from getting too dark.
Remove cake from the tin and let cool down completely on a cooling rack.
Melt chocolate for the glaze in a double broiler and and spread over cooled cake. Let set before cutting.