Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.

In a medium bowl, whisk together the flour and cocoa powder until well combined.

In a large bowl, beat the butter and sugar until light and fluffy. Add the egg and vanilla extract, mixing until fully incorporated.

Gradually mix the dry ingredients into the wet mixture until just combined. Optionally, melt the chocolate chips and stir in for added flavor.

Drop heaping tablespoons of dough onto the baking sheet, spacing them about 2 inches apart.

Bake for approximately 10 minutes, until the tops are set. Let cool on a wire rack.

Beat together butter and powdered sugar for the frosting, then add vanilla and milk until smooth.

Once cookies are cool, pipe the frosting onto each and decorate with chocolate drizzle and candies.