In a large mixing bowl, beat together 1 cup of softened unsalted butter and 1 cup of granulated sugar using an electric mixer on medium speed for about 3 minutes until pale and fluffy. Add in 1 large egg and 2 tablespoons of lemon zest, mixing until fully combined.

In a separate bowl, whisk together 2.5 cups of all-purpose flour, 2 teaspoons of cornstarch, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt. Gradually stir the dry mixture into the wet ingredients until a thick dough forms, ensuring not to over-mix.

Gently fold in 1/2 cup of finely chopped fresh raspberries into the dough. Be careful to preserve the vibrant color while ensuring they are well incorporated.

Drop small spoonfuls of 3 tablespoons of raspberry jam onto the cookie dough and lightly swirl it in using a knife or skewer, maintaining the eye-catching swirls.

Using a cookie scoop, portion the dough into rounded mounds on a lined baking sheet, placing them about 2 inches apart.

Preheat your oven to 350°F (175°C) and bake for 10-12 minutes until the edges are pale golden and the centers are slightly underbaked.

Let the cookies cool on the baking sheet for about 5 minutes, then dust the tops with powdered sugar before serving.