Dough: Preheat oven to 350F (180 C).
In medium bowl, whisk together flour and baking powder.
In bowl of stand mixer fitted with the paddle attachment, cream butter, granulated sugar and vanilla sugar for 4 minutes or until light and fluffy. Add flour mixture and mix for 5 minutes or until a soft dough forms. Shape dough into a ball; cover with plastic wrap and refrigerate for 30 minutes or until chilled.
On lightly floured work surface, roll out dough to 1/4-inch (5 mm) thickness. Using 3-inch (7.5 cm) egg-shaped cookie cutter, cut out cookies and arrange on parchment paper–lined baking sheet(s).
Bake for 11 minutes or until cookies are set. Transfer the parchment paper with the cookies to a wire rack to cool completely.
Decorate: Divide royal icing into 4 even portions. Add 3 to 5 drops of green food colouring, as desired, to 1 portion and blend in colour. Tint the other portions yellow, red and blue. Between 2 sheets of parchment paper, roll out each portion to 1/4-inch (5 mm) thickness. Using the egg-shaped cookie cutter, cut out royal icing eggs.
Top each cookie with a fondant egg. Use icing pens to decorate cookies.