Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Cream the softened butter and sugar in a large bowl until light and fluffy.

Beat in the egg and vanilla extract until the mixture is smooth and glossy.

Whisk together flour, cocoa powder, baking soda, and salt in a separate bowl.

Gradually mix dry ingredients into the wet mixture until just combined.

Add milk one tablespoon at a time if the dough is too dry.

Roll the dough into 1-inch balls and place 2 inches apart on prepared sheets.

Bake for 10-12 minutes until cookies are set and edges are firm.

Press a chocolate kiss into the center of each cookie immediately after removing from the oven.

Cool on the baking sheet for 5 minutes before transferring to a wire rack.