Make the cookie dough:
Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large mixing bowl, cream the softened butter and powdered sugar until light and fluffy. Mix in the vanilla extract.
Gradually add the flour and cornstarch to the butter mixture, beating until a soft dough forms.
Shape and bake:
Scoop tablespoon-sized portions of dough and roll into balls. Place them on the prepared baking sheet about 2 inches apart.
Flatten each ball slightly with the back of a spoon or your palm.
Bake for 10-12 minutes, or until the edges are just beginning to turn golden. Allow cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack.
Prepare the glaze:
In a small bowl, whisk together the powdered sugar, raspberry jam, and milk or cream until smooth. Adjust the consistency by adding more powdered sugar or milk, as needed.
Decorate the cookies:
Once the cookies are completely cooled, drizzle or spread the raspberry glaze over the tops using a spoon or piping bag. Allow the glaze to set for about 15 minutes before serving.