Make the cookie dough

1. Cream the softened butter and sugars until fluffy.
2. Mix in the egg and vanilla until smooth.
3. In a separate bowl, combine flour, cocoa powder, baking soda, and salt.
4. Add the dry ingredients into the wet ingredients and mix until no dry flour remains.
5. If the dough is sticky, chill it in the fridge for 20 to 30 minutes.

Bake the cookies

1. Preheat the oven to 350°F (175°C).
2. Scoop dough into balls and place on a lined baking sheet with space between them.
3. Bake for 9 to 11 minutes, until edges are set but centers are still slightly underdone.
4. Cool on the baking sheet for a few minutes before transferring to a wire rack.

Make the frosting

1. Beat softened butter with powdered sugar.
2. Add milk or cream and mix until desired consistency is reached.
3. Add green food coloring to achieve a vibrant slime effect.

Assemble the cookies

1. Ensure cookies are fully cool before filling.
2. Pair cookies by size and spread a thick ring of green frosting on one cookie, adding extra in the center.
3. Press the second cookie on top, allowing the filling to peek out at the edges.

Decorate

1. Press M and M candies onto the cookies right after baking or after filling.