Beat together the butter, sugar, and eggs until creamy.

Add in the flour, baking powder, salt, and vanilla. Beat until smooth.

Roll out the dough between 2 sheets of parchment paper (1/4-1/2" thick).

Place dough on sheet tray and place in refrigerator to chill for at least one hour.

Remove tray from fridge and cut out cookies. Place cutouts on ungreased cookies sheets and place back in the fridge for at least 1 hour.

Preheat oven yo 375° F.

Bake cookies at 375° for 5-8 minutes, until they just start to brown.

Cool cookies on baking sheet for 10 minutes before moving them to cooling racks.

Cool completely before icing.

Pour the egg whites in the bowl of an stand mixer with the whisk, mix on medium until egg whites are frothy.

Add the confectioner's sugar and vanilla beat on low speed until blended. Increase to medium for 5-7 minutes until it is thick and shiny.

If it seems too thick, doesn't reincorporate when dropped back into bowl, add water one half tablespoon at a time until it reached the desired consistency. Don't rush this step, it takes time.

Divide the icing into separate bowls for each color you are planning on using. Start with just 1-2 drops of color first. Mix it in completely then add more if needed.