Incorporate egg, vanilla extract, and red gel color and mix until well combined. Scrape the bottom and sides of the bowl and mix. Add the dry ingredients to the wet ingredients and mix at low speed until just combined. Do not overmix.
Divide the dough into 2 equal dough balls. Lightly dust the rolling pin, then roll out the first disc of dough over a silicone mat to a thickness of 1/4″. You can also lightly dust your work surface to roll the dough there instead.
Use a 2.5" square cookie cutter and a 1.3" heart cookie cutter to cut out your cookies. Create one square-shaped cookie and another square-shaped cookie with a heart-shaped hole in the center (which will be used as the top cookie). Arrange them on the prepared baking sheets, leaving 1.5 inches of space between each.
Bake for 10-11 minutes. Remove from the oven and let cookies cool for a few minutes on the cookie sheet, then transfer them to a wire rack to cool completely before assembling them
Cream Cheese filling
Over a large mixing bowl of an electric mixer, cream the butter at medium speed for 3 minutes until light and fluffy. Incorporate the cream cheese and beat for an extra minute.
Add the sifted powdered sugar, vanilla extract, and salt to the cream cheese mixture, and mix for an additional 1-2 minutes. Do not overbeat. To assemble your cookies, place the cream cheese filling in a piping bag with a 1M Wilton tip.