With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy – about 2 minutes
Add sugar and beat until smooth.
Add eggs one at time and beat after each egg until smooth. Add the vanilla with the last egg.
Mix in flour, baking powder and salt. Blend well until combined.
Divide dough in half and wrap with plastic wrap. Chill in refrigerate for 1 - 3 hours.
Roll dough onto floured surface until desired thickness.
Cut out your number or design. Reshape the remaining dough and repeat for as many layers as you want. Bake one layer on their own cookie sheet.
Bake layers at 350 degrees for 8 -10 minutes or until golden brown. It depends on your thickness of the cookie and it can take longer to bake.
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy – about 2 minutes
Add 1 cup of confectioners' sugar and 1 tbsp milk at a time. Beat on low speed for 30 seconds after each cup of confectioners’ sugar and tablespoon of milk is added, then increase to medium-high speed and beat for 2 full minutes after the 4th cup is added. Add up to 1 cup more confectioners’ sugar if frosting is too thin or another tablespoon of milk if frosting is too thick. Once you've reached the desired consitency, mix in the vanilla.
Assemble each layer of your cookie cake and decorate as desired.