Step 1 — Make the shortbread base
Preheat the oven to 350°F (175°C) and line a baking sheet with parchment. In a mixer, beat the softened butter with the granulated sugar until the mixture turns pale and fluffy — about 2–3 minutes. Add the powdered sugar and mix briefly for extra silkiness. Beat in the egg and vanilla until fully incorporated.

Step 2 — Add the dry ingredients
Sprinkle in the salt and the optional baking powder, then add the flour gradually on low speed. Mix just until the dough forms and no streaks of flour remain. Stop mixing once combined — overworking will make the cookies tough.

Step 3 — Prepare the raspberry swirl
If using jarred jam, warm it slightly so it spreads easily. If you prefer fresh raspberries, blitz them into a purée and press through a fine mesh to remove seeds; add a teaspoon of sugar if the purée tastes too tart. You want a fairly thick spread so the swirl holds up during rolling and baking.

Step 4 — Roll, spread, and shape
Turn the dough out onto a lightly floured surface and roll it into a rectangle about ¼ inch thick. Spread an even, thin layer of raspberry jam over the dough, leaving a small clean border around the edges. Roll the dough tightly into a log (like a jelly roll), then wrap it in plastic wrap.

Step 5 — Chill and slice
Refrigerate the log for at least 1 hour so it firms up and slices cleanly. Once chilled, remove the wrap and cut the log into rounds approximately ¼ inch thick. Arrange the rounds on the prepared baking sheet with a little space between each.

Step 6 — Bake and cool
Bake the cookies for 10–12 minutes, or until the outer edges take on a faint golden hue. Watch closely — ovens vary. Let the cookies rest on the sheet for about 5 minutes so they set, then transfer them to a wire rack to cool completely.