In a large mixing bowl, cream the butter and powdered sugar until smooth and fluffy.

Add the lemon zest and vanilla extract, mixing until just combined.

Whisk together flour and salt, then add gradually to the butter mixture. Mix until a soft dough forms.

Transfer dough to parchment paper. Flatten slightly and spread black raspberry preserves in thin streaks over the surface. Gently fold or roll the dough to create swirls, without overmixing.

Shape into a log, wrap in parchment, and chill for at least 1 hour (or overnight).

Preheat oven to 350°F (175°C). Slice chilled dough into 1/4-inch rounds and arrange on a parchment-lined baking sheet.

Bake for 10–12 minutes, until edges are just turning golden.

Cool on the sheet for 5 minutes, then transfer to a wire rack to cool completely.