The traditional British filling for these swirl sandwich cookies is raspberry or strawberry jam with buttercream frosting. Our cookies are made with homemade raspberry filling and a white chocolate mascarpone cream. These shortbread cookies look impressive, and they're easy to make if you follow this simple recipe trick: Refrigerate them before baking to re-solidify the butter so the cookies hold their shape in the oven.
a piping bag fitted with a medium open star tip; a piping bag fitted with a small round tip
For the raspberry filling: Combine the raspberries, granulated sugar and lemon juice in a medium saucepan and heat over medium heat, crushing the fruit and stirring occasionally. Bring the mixture to a boil, whisk in the cornstarch and lower to a simmer. Simmer until slightly thickened, about 5 minutes. Remove from the heat and allow to cool to room temperature. Refrigerate until ready to use.
For the cream filling: Add the white chocolate to a bowl fitted over a pot of simmering water to make a double boiler. Heat until the chocolate is fully melted, then remove the bowl and allow to cool slightly, 5 to 10 minutes.
Once cool, whisk in the mascarpone and confectioners' sugar and continue to whisk until all lumps of sugar are dissolved and the mixture is smooth and velvety.
For the cookie dough: Preheat the oven to 350 degrees F.
Add the butter to the bowl of a stand mixer fitted with a paddle attachment and beat until light and fluffy, 3 to 4 minutes. Add the confectioners' sugar and vanilla to the bowl and continue to beat, scraping down the sides of the bowl as needed, until well incorporated, 60 to 90 seconds.
Whisk together the flour and salt in a medium bowl. On low speed, add the flour mixture to the butter mixture and beat to combine. Add the milk and mix just a bit more until everything is incorporated. Transfer the dough to a piping bag fitted with a medium star tip.
Line 2 baking sheets with parchment and pipe 1 1/2-inch circles of dough, about 12 on each sheet. Chill in the refrigerator for about 8 minutes.
Bake until light golden brown on the sides, 12 to 15 minutes. Allow to cool on the baking sheets.
Once all cookies have been baked and cooled, spoon the raspberry filling onto the bottom of half of the cookies. Scoop the cream filling into a piping bag fitted with a small round tip and pipe it on top of the raspberry filling. Use the remaining cookies as tops to create sandwich cookie.