Crush the thin mint cookies in either a food processor, or sealed plastic ziplock bag. Pour the crushed cookies into a shallow dish, and prepare your milkshake glasses by lighting moistening the edges then placing into the crushed cookies. Gently twist the glass to be sure to evenly coat the rimmed edge.
Place the creme de menthe, creme de cacao, and ice cream into a blender. Blend until thick and creamy.
Pour into 6 ounce glasses. Using a mini cookie scoop, add an additional scoop of ice cream and garnish with a sprig of mint. (both are optional)