Preheat oven to 350°F and line baking sheets with parchment paper.
Beat butter and powdered sugar until fluffy. Mix in egg yolk and vanilla.
Add flour, cornstarch, and salt. Stir in milk until soft dough forms.
Pipe small cookie swirls onto baking sheets and chill for 20 minutes.
Bake for 10–13 minutes until lightly golden. Cool completely.
Melt chocolate chips with coconut oil until smooth.
Dip half of each cookie in chocolate and sprinkle with chopped nuts.
Let chocolate set before serving.
