For the jam, place the raspberries in a small, deep-sided saucepan and crush them with the back of a wooden spoon or masher. Add the sugar and lemon juice and bring to boil over a low heat. When the sugar has dissolved, increase the heat to medium and boil for about 4 minutes. Remove from the heat and carefully pour into a shallow container (pass it through a fine mesh sieve if you prefer a seedless jam). Set aside to cool completely until ready to use.

For the cookies, preheat the oven to 375°F. Line two baking sheets with parchment paper. Using a 2-inch round cookie cutter as a guide, draw 12 circles on each paper, spaced well apart. Turn the paper over so the pencil marks are underneath. Set aside.

Beat together the butter and powdered sugar in a large bowl until pale and fluffy, about 4 to 5 minutes. Sift in the flour and cornstarch and beat well, until thoroughly mixed. Transfer the mixture into a large piping bag fitted with a medium star nozzle. Pipe 24 swirled rounds (not rosettes), inside the circles on the prepared baking sheets.

Bake the cookies until pale golden-brown in color, about 13—15 minutes. Remove from oven and cool on the baking sheets for 5 minutes then carefully transfer to a wire rack to cool completely and harden.

For the filling, beat the butter in a bowl and sift in the powdered sugar. Add the vanilla extract and beat on moderate speed until very light and smooth. Spoon into a piping bag fitted with a large star nozzle. Spoon a little jam onto the flat side of 12 of the cookies and place jam-side up on a baking sheet. Pipe the buttercream over the jam and sandwich with the remaining cookies. Serve immediately or place in an airtight container and store in the fridge for up to 4 days. Allow to come to room temperature before eating. Enjoy!