Step 1: In a large bowl, cream together the softened unsalted butter and 1 1/2 cups of granulated sugar with an electric mixer on medium-high speed until light and fluffy, about 2-3 minutes.
Step 2: Beat in the large egg until just combined. Then, add the 2 teaspoons of vanilla extract, zest of 2 large lemons, and 1 teaspoon of fresh lemon juice. Mix until well incorporated, scraping down the sides of the bowl as needed.
Step 3: In a separate medium bowl, whisk together the 3 cups of all-purpose flour, 1 teaspoon of baking powder, and 1/2 teaspoon of salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix. Stop as soon as no streaks of flour remain.
Step 5: Divide the cookie dough roughly in half. In a small saucepan, combine the 1 cup of raspberries, 1 tablespoon of granulated sugar, and 1 teaspoon of fresh lemon juice. Cook over medium heat until the raspberries break down and form a thick sauce, about 5-7 minutes. Press the raspberry mixture through a fine-mesh sieve to remove seeds, then let the puree cool completely.
Step 6: Once cooled, mix the raspberry puree into one half of the divided cookie dough until evenly colored and combined.
Step 7: Lay out a piece of plastic wrap. Place the plain lemon dough on one side and the raspberry dough on the other. Gently press them together. You can either layer them and then gently roll, or dollop alternating spoonfuls onto the plastic wrap and then gently roll it into a log. The goal is to create a marbled effect, not to fully combine the colors.
Step 8: Tightly wrap the log of swirled dough in plastic wrap and chill it in the refrigerator for at least 2 hours, or up to 2 days, until firm.
Step 9: Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. Remove the chilled dough log from the refrigerator and use a sharp knife to slice it into 1/4-inch thick rounds. Arrange them on the prepared baking sheets, leaving about 2 inches between each cookie.
Step 10: Bake for 10-12 minutes, or until the edges are lightly golden and the centers are just set.
Step 11: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Store in an airtight container at room temperature for up to 3-4 days.