Using an electric mixer, cream the butter, cream cheese and sugar on high speed for 3 minutes.
Add the vanilla, egg yolk and salt and mix at low speed until blended. Mix in the flour at low speed until incorporated.
Halve the dough; pat into 2 disks. Wrap in plastic wrap; refrigerate for 30 minutes.
Preheat the oven to 375 degrees . Line 2 cookie sheets with parchment. On a floured surface, roll out 1 piece of dough 1/4 inch thick, dusting with flour if the dough sticks.
Using a 3-inch Christmas tree cutter, cut out cookies and transfer to the prepared pans. Gather the dough scraps and knead into a disk. Refrigerate the dough for 15 minutes before rolling out more cookies.
Sprinkle the cookies with the sanding sugar. Bake until the edges are golden, 12 to 15 minutes. Using a thin metal spatula, transfer the cookies to racks to cool. Repeat with the remaining dough and sugar.