Make the Filling: In a medium saucepan, combine 1 cup of raspberries, 3 tbsp granulated sugar, 1 tbsp cornstarch, and 1 tsp lemon juice. Cook over medium heat, stirring gently, until the mixture thickens, which should take about 5-7 minutes. If you prefer a smooth filling, strain the mixture through a fine mesh sieve to remove the seeds. Allow it to cool completely.

Prepare Cookie Dough: In a large mixing bowl, whisk together 2 ½ cups all-purpose flour, ½ tsp baking powder, and ¼ tsp salt. In a separate bowl, beat ¾ cup softened unsalted butter and 1 cup granulated sugar together until fluffy and light in color. Add in 1 large egg, 1 tsp vanilla extract, and 2 tbsp milk, and mix until well combined. Gradually add the dry ingredients into the wet ingredients, mixing until a soft dough forms.

Assemble & Chill: Divide the cookie dough into two equal portions. On a piece of parchment paper, roll each portion into a rectangle roughly ½ inch thick. Spread a thin layer of the cooled raspberry filling over each rectangle, leaving about a ½-inch border. Tightly roll up each rectangle into a log shape and wrap in plastic wrap. Refrigerate the logs for at least 2 hours to firm up.

Slice & Bake: Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. Once the dough is firm, remove the logs from the fridge and slice them into ½-inch thick rounds. Place the slices on the prepared baking sheets, spacing them about 2 inches apart. Bake for 10-12 minutes or until the edges are golden brown. Remove from the oven and let cool on wire racks.