Cookies: Preheat your oven to 375° and line three baking sheets with parchment paper.  You can use a round cutter to trace 2 inch circles onto the bottom side of the parchment, or you can freehand the circles when you pipe them on. In a mixing bowl, use a hand mixer to beat together the cup of butter with the 1/2 cup of powder sugar until light and fluffy.  Sift in the flour and cornstarch, then beat until fully incorporated. Fit a piping bag with the coupler and a large star tip (like this) and fill with the dough.  Pipe 2″ rounds onto the parchment paper.  If the dough is extremely soft, refrigerate it for 15 minutes to help the dough maintain it’s shape in the oven, then bake for 12-15 minutes or until they are golden and very lightly browned around the edges. Filling: Beat together the 2 cups of powdered sugar, 7 Tbsp. butter, and 1/2 tsp. vanilla.  Spoon into another piping bag fitted with coupler and star tip.  Pipe the buttercream onto the flat side of half of the cookies, and spread about 1/2 tsp. of jam onto the other half.  Sandwich them together and refrigerate until the buttercream is firm.  Store in an airtight container in the refrigerator for 2-3 days.