Start by combining flour, cornstarch, and salt in a medium bowl. Then, beat the butter and sugar in a stand mixer until light and fluffy. This ensures your cookies have the classic melt-in-your-mouth texture.
Transfer the dough into a pastry bag fitted with a star tip. Pipe swirls onto parchment-lined baking sheets, keeping them evenly spaced. The swirls give the cookies their beautiful, signature look.
Bake the cookies at 350°F (175°C) for 12–15 minutes until lightly golden at the edges. Allow them to cool completely on a wire rack before filling. Cooling is key to prevent the buttercream from melting.
Beat butter until smooth, then gradually add confectioners’ sugar, milk, vanilla bean paste, and salt. Mix until creamy and pipeable. The buttercream should hold its shape but still be soft enough to spread easily.
Spread or pipe a small amount of buttercream on the flat side of one cookie, add a teaspoon of raspberry jam, and sandwich with another cookie. Press gently to combine. Repeat until all cookies are filled.