Add the ice cubes, milk, sugar, and vanilla extract to your blender, then secure the lid.
Blend on high until the mixture is thick, icy, and smoothly blended, about 30-40 seconds. You should see a frosty texture forming.
Add the bittersweet chocolate chunks and mint leaves (if using) into the blender.
Pulse a few times to evenly distribute the chocolate and mint without fully breaking down the ingredients, preserving some chunks for texture.
Check the consistency—if it's too thick, add a splash more milk and blend briefly until smooth but still icy.
Pour the slushie into glasses, revealing its thick, icy, and slightly creamy appearance with bits of chocolate and fresh mint visible.
Optional: Garnish with additional mint leaves or chocolate shavings for an attractive finish.
Serve immediately with a straw or spoon, enjoying the cool, crunchy texture and burst of flavors.