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

In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract, mixing until smooth.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually mix the dry ingredients into the wet ingredients. Stir in the milk to keep the dough soft.

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

Bake for 8–9 minutes until slightly puffed but not fully set.

Remove from the oven and quickly press a half marshmallow into the center of each cookie.

Return to the oven for 2–3 minutes until marshmallows are soft and puffy.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.

Melt the chocolate chips and vegetable oil in a microwave-safe bowl in 30-second intervals until smooth.

Drizzle or spoon the melted chocolate over each cookie and garnish as desired.