Preheat the oven to 160°C (approximately 320°F). Grease a springform pan with butter.
For the cake: Beat butter, sugar, vanilla sugar and salt until creamy. Separate eggs and gradually beat yolks into butter mixture. Combine flour and baking powder. Gradually add to butter mixture, alternating with advocaat. Beat egg whites until stiff and fold into batter.
Pour batter into pan, flatten and bake for about 50 minutes. Remove from oven and allow to cool in the pan. Release from pan once cooled.
For decoration: Zest an orange. Briefly blanch orange and pat dry. Squeeze some juice from orange. Combine powdered sugar, advocaat and some orange juice to form a thick paste. Spread onto cake and sprinkle with orange zest. Allow to dry, slice and serve.