Preheat oven to 350°F (175°C).
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Beat in the vanilla extract and eggs until well combined.
In another bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Drop tablespoon-sized balls of dough onto a baking sheet.
Bake for 10 minutes, then let cool on a wire rack.
For the frosting, beat together powdered sugar, butter, peppermint extract, and milk until smooth.
Frost the cooled cookies and sprinkle with crushed peppermint candies.