Cream the butter and powdered sugar with an electric mixer for about 3 minutes until light and fluffy. Beat in vanilla and egg yolk until smooth. Add flour and salt, mixing just until combined—don’t overmix.

Gently fold in mini chocolate chips and all but 1 tablespoon of Halloween sprinkles.

For slice-and-bake cookies: Shape dough into a 10-inch log, wrap in plastic, and chill for 2 hours. Slice into ¼-inch rounds and place 1 inch apart on lined baking sheets.

For cut-out cookies: Divide dough, roll each half between floured parchment to ¼-inch thick, and press in extra sprinkles if desired. Chill for 2 hours. Cut shapes, transfer to baking sheets, and chill again for 20 minutes.

Bake at 350°F for 10–12 minutes, until edges are just golden. Let cool on tray for 10 minutes, then transfer to wire racks. If desired, melt chocolate chips, transfer to a zip-top bag, and pipe designs on cookies once cooled.