In a mixing bowl, combine all-purpose flour and cubed unsalted butter, mixing until the mixture resembles coarse crumbs. Gradually add heavy cream, stirring until the dough forms. Shape the dough into a disk, wrap it in plastic wrap, and chill in the refrigerator for at least 1 hour, or until firm to the touch.

Preheat your oven to 375°F (190°C). On a floured surface, roll out the dough to about 1/8 inch thick. Use a round cookie cutter to cut out 1½-inch circles. Coat each cookie round in crystal sugar, ensuring they are covered on both sides, and arrange them on parchment-lined baking sheets.

Place the baking sheets in the oven and bake the cookies for 8-10 minutes, or until slightly puffed and edged in golden brown. Remove cookies from the oven and let them cool completely on the sheets for about 10 minutes before transferring to a wire rack.

In a mixing bowl, beat the softened butter until creamy. Gradually add sifted powdered sugar and vanilla extract, mixing until smooth and fluffy. If the buttercream is too thick, add a splash of heavy cream. Divide the buttercream into two bowls and color one red and the other green.

Use a piping bag to pipe a generous amount of the colored buttercream onto the flat side of half the cookies. Top each frosted cookie with another cookie, gently pressing down to create a sandwich.