Beat butter, granulated sugar, and Rodelle Gourmet Vanilla Extract in a mixer bowl on medium speed, until smooth. On low speed, beat-in the egg yolk. Add flour, cornstarch, and salt. Mix on low speed with paddle attachment until dough comes together in a ball. Wrap the ball in plastic and refrigerate for about 30 minutes.
Divide the dough in half. Dust a rolling surface and your rolling pin with flour. Roll each portion to about 1/8 inch thickness.
Using a 2 to 3-inch star shaped cutter, cut out the cookies. Use a ½ - inch cutter to cut out the centers of half the cookies to make a window for the jam filling. Bake the cut-outs separately, or gather the dough and roll to form more cookies for sandwiches.
Place the cookies on baking sheets about 1 inch apart and bake until edges are just beginning to brown, about 10 to 12 minutes. Rotate baking sheets in oven if using more than one at a time.
Transfer to wire racks to cool completely.
To form sandwich, spread the flat side of the whole cookies with about ½ teaspoon of jam. Top each with a cut-out cookie, flat side toward the filling. Sprinkle the tops with powdered sugar.
These cookies are best eaten the day they are filled, or store unfilled cookies in an airtight container for 3 days or freeze for longer.
