Place cake pan holding batter in 23 x 15 cm cake pan. Tuck cardboard between 2 pans. Bake till middle of cake doesn't make squishing sound when pressed gently, about 55 minutes. Remove pans from oven.Remove cardboard and outer pan. Drop pan holding cake from about 30 cm high, 2-3 times. Invert pan onto wire rack. Leave till just cool. As soon as cake is cool, slide knife along sides of pan. Unmould cake. Discard both layers of parchment paper.