Heat the oven to 350°F. With a wooden spoon or a mixer, beat the butter, cream cheese, and sugar until very smooth, scraping down the sides of the bowl often (if you’re using an electric mixer, set it on medium speed). Stir in the vanilla. Add the flour; mix until just combined. Fit a pastry bag with a wide star tip and fill the bag with the dough. On parchment-lined cookie sheets, pipe the dough in 1-1/2-inch “S” shapes about 11/2 inches apart. Press your thumb into each end of the “S”, leaving a small indentation. Bake until light brown around the edges, 17 to 19 minutes. Transfer the cookies to a rack to cool completely. Fill each indentation with about 1/8 tsp. of preserves.
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