Line the springform tin completely with baking paper.
Beat softened butter, sugar, lemon zest, vanilla and salt for approx. 5 minutes until light and creamy.
Add eggs one at a time and beat each egg briefly to form a homogeneous mixture.
Pour in milk and mix briefly.
Mix flour with cornstarch and baking powder.
Add the flour mixture to the butter-egg-milk mixture and mix briefly but thoroughly to form a homogeneous, airy, thick batter.
Divide the batter into 6 equal portions into 6 clean bowls.
Color the batter portions in purple, blue, green, yellow, orange and red. To do this, add about 3 teaspoons of fruit powder or tasteless vegetable powder to each portion of batter.
First place one of the batter portions in the springform tin, smooth it out and bake the cake base in a preheated oven at 338 °F (170 °C) for approx. 15 minutes. Then carefully remove the cake base from the springform tin together with the baking paper. Line the springform tin with clean baking paper again and bake the next cake bases one after the other in the same way.
Leave all 6 cake bases to cool and remove the baking paper.