preheat the oven to 375˚
line two baking sheets with parchment, draw 28 1 1/2 inch circles on the parchment, and then flip it over so you don't get it on your cookies--these are piping guides you don't HAVE to pipe with them
beat the butter and powdered sugar for the cookies together until light and fluffy
add the flour, cornstarch, and a pinch of salt to the bowl and beat again until the dough is very light
put it in a piping bag fitted with the large star tip-really any size tip will work here the opening on mine is less than an inch and it worked well
pipe 28 whirls on your prepared baking sheets, just pipe a circle and then make sure the center is filled in
bake the cookie for 10-12 minutes, take them out before they brown up, don't let them burn
cool on the baking sheets
meanwhile whip the butter, powdered sugar, and vanilla together until light and fluffy
load the buttercream in a piping bag fitted with whatever tip you want to use
when your cookies are completely cool set them in pairs according to size and shape
then pipe a generous dollop of buttercream on the bottom halves
top with a scoop of jam
gently press the top on and set on a serving plate, be careful these cookies are VERY DELICATE and will be crushed easily