Preheat oven to 170°C. Line a baking sheet with parchment paper.
Whip the butter with the sugar until creamy.
Add eggs and vanilla extract and mix well.
Add flour and baking powder and mix until just combined.
Take about 1/3 of the batter out and transfer to another bowl.
Add the desired colouring for the surprise inside and mix until you have an even colour.
Pour the batter on the prepared baking sheet and smooth out. You can simply pour it on the baking sheet or use a baking dish or a mold.
Bake ca. 10 minutes.
Take out of the oven and let cool slightly.
Meanwhile, grease a cake tin and/or line it with parchment paper.
If desired, you can colour the rest of the batter as well.
Once the sheet cake has cooled enough to handle, cut out the forms.
Now carefully place all cut out pieces in a straight line on the dough layer. Lightly press it down.
Fill up with the remaining batter.
Bake ca. 50-60 minutes or until a tooth pick inserted in the middle comes out clean.
Let cool completely before slicing.