In a bowl, beat the softened butter with a whisk or hand mixer. Sift in the powdered sugar and cream together until light in color.

Add the egg yolks and beat together with the butter and sugar until fully incorporated. Add the vanilla and whisk together.

Sift in the flour and fold it in, being careful to not overmix. Continue to incorporate the dough together by lightly kneading.

Roll the cookie dough between two sheets of parchment paper to 5mm thickness and freeze for 10 minutes. Once the dough is firm, cut shapes with cookie cutters, a knife, or a pastry wheel. Freeze the cut-out cookie shapes for 10 minutes.

Arrange the cookies onto a baking sheet lined with parchment paper and bake in an oven at 340°F/170°C for 12-15 minutes until the edges are lightly golden brown. Allow the cookies to cool completely on a cooling rack.

While the cookies cool, prepare the icing. In a bowl, beat together the powdered sugar and egg whites. Once it’s about half incorporated, add the lemon juice and vanilla and continue to mix until smooth. The icing should be thick but still thin enough to flow freely.

Prepare the colors for the icing and decorate as you like. To have colors next to each other flat, pipe wet icing on wet icing. To have some icing 3D, pipe wet icing on dry, hard icing. Be mindful to let the icing harden completely, and the frosted cookies are done! 完成です！