Cream together the unsalted butter and superfine sugar until light and fluffy. Add the egg, egg yolk, and vanilla, mixing until incorporated. Sift in cake flour and salt, blending until combined.

Wrap the dough tightly in plastic wrap and chill in the refrigerator for 40 minutes to 1 hour.

Preheat the oven to 160°C (320°F). Roll out the chilled dough to ¼ inch thick and cut into shapes. Bake for 15 minutes until edges are lightly golden, then cool on a wire rack.

For the frosting, beat together softened butter and powdered sugar until crumbly. Add milk and vanilla, beating until light and fluffy.

Color the buttercream as desired and pipe onto cooled cookies to create rose designs.