In the bowl of an electric mixer, beat the butter, cream cheese, sugar and salt until light and fluffy. Add the egg and vanilla and continue to beat until combined. Gradually add the flour and beat until a dough forms. Cover and refrigerate for 2 hours or until firm.

Preheat the oven to 350 degrees.

On a floured surface, roll out the dough using a rolling pin to a 1/8 inch thickness. Cut the dough with cookie cutters dipped in flour. Place the cutouts on a baking sheet lined with parchment paper.

Bake for approximately 7-8 minutes or until the edges begin to brown. Cool on the pan for 1 minute and then transfer to a cooling rack to cool completely.

In the bowl of an electric mixer, mix the egg whites and vanilla until frothy. Gradually, add the sugar and mix on low speed until the sugar is incorporated and the icing is shiny. Turn the mixer to med-high and beat until the mixture forms stiff, shiny peaks, approximately 5-7 minutes.

Stir in food coloring if desired.

For immediate use, transfer to a pastry bag fitted with a tip and pipe onto the cookies. (Or if you're using a zip top bag, clip corner and pipe onto cookies.)

Icing can be stored in an airtight container for up to 3 days.