Preheat the oven to 180°C. Line baking sheets with parchment paper.

For the cookies: in a bowl, whisk together flour, baking powder, and salt. Set aside.

In another bowl, beat butter and sugar until light and fluffy. Add the egg and vanilla extract, and beat until combined.

Gradually add flour mixture, beating until just combined.

Roll out the dough on a floured surface, cut into mermaid shapes using cookie cutters, and place them on the baking sheets.

Bake the cookies for 8-10 minutes or until the edges begin to turn golden. Cool completely.

For the royal icing: in a bowl, beat powdered sugar, egg white, and lemon juice until stiff peaks form. Divide icing into bowls and color with food coloring.

Decorate the cooled cookies with the icing and let them set.

For the cake: grease and flour two 20 cm round cake pans.

In a bowl, whisk together flour, baking powder, and salt. Set aside.

In another bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition, then add vanilla extract.

Alternately add the flour mixture and milk, beginning and ending with flour. Mix until just combined.

Divide the batter between the prepared pans and bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Cool the cakes completely.

For the buttercream frosting: beat butter until creamy. Gradually add powdered sugar, beating until light and fluffy.

Beat in vanilla extract and heavy cream until smooth. Tint with blue food coloring.

Assemble the cake by spreading a layer of frosting between the cake layers, then frost the top and sides of the cake.

Arrange the decorated cookies on top of the cake.