Cream the Base: In a large mixing bowl, beat the softened butter and granulated sugar together for about 3 minutes, until light, fluffy, and pale in color. This step incorporates air into the dough, giving the cookies their tender texture. Add the egg and vanilla extract and beat until fully combined and smooth.
Add the Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture on low speed, mixing just until a soft, cohesive dough forms. Avoid overmixing to keep the shortbread tender.
Roll Out the Dough: Divide the dough evenly in half. Place each portion between two sheets of parchment paper and roll into a rectangle about ¼-inch thick. Keeping the dough between parchment prevents sticking and makes rolling easier.
Prepare the Jam: In a small bowl, whisk the raspberry jam with the cornstarch until smooth. This step thickens the jam slightly, helping it stay neatly inside the swirl without leaking out during baking.
