Prepare the Raspberry Filling:

1. Cook the Filling: In a small saucepan, combine the raspberries, sugar, cornstarch, and lemon juice. Cook over medium heat, stirring constantly, until the mixture thickens and resembles jam, about 5-7 minutes.
2. Strain (Optional): If you prefer a seedless filling, press the mixture through a fine-mesh sieve to remove seeds.
3. Cool Completely: Transfer the filling to a bowl and let it cool to room temperature.

Make the Cookie Dough:

4. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt.
5. Cream Butter and Sugar: In a large mixing bowl, beat the butter and sugar with an electric mixer until light and fluffy, about 2-3 minutes.
6. Add Wet Ingredients: Beat in the egg, vanilla extract, and milk until combined.
7. Combine Dry and Wet Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.

Assemble and Chill:

8. Roll Out the Dough: Divide the dough into two equal portions. Roll each portion into a rectangle, approximately ¼ inch thick, on a sheet of parchment paper.
9. Spread the Filling: Spread a thin, even layer of the raspberry filling over each rectangle, leaving a ½-inch border around the edges.
10. Roll the Dough: Starting from one long side, roll the dough tightly into a log. Wrap the logs in plastic wrap and refrigerate for at least 2 hours or until firm.

Slice and Bake:

11. Preheat the Oven: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
12. Slice the Cookies: Using a sharp knife, slice each log into ½-inch thick cookies and arrange them on the prepared baking sheets, spacing them about 2 inches apart.
13. Bake: Bake for 10-12 minutes, or until the edges are lightly golden.
14. Cool: Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.