Chop the chocolate and melt with the butter over a hot water bath, stirring constantly.
Separate the eggs. Beat the egg whites and lemon juice until very stiff.
Beat the egg yolks, sugar, lemon zest and vanilla until fluffy. Slowly stir in the chocolate mixture and the almonds.
Fold the egg whites into the chocolate mixture. Sift in the cornstarch and loosely fold together.
Grease the pan with butter and sprinkle with flour. Pour in the dough and smooth.
Bake for 45 minutes at 160°C (approximately 325°F), until a toothpick inserted into the center comes out clean. Remove from the oven and cool slightly. Remove from the pan and cool completely on a wire rack. Dust with powdered sugar and cocoa. Cut into pieces and serve.