Add the crushed ice to your blender, ensuring it's evenly distributed in the container.
Pour in the milk, then add the unsweetened cocoa powder and sugar directly on top of the ice.
Toss in the vanilla ice cream for extra creaminess and a more decadent texture.
Seal the blender and blend on high speed for about 30-45 seconds, until the mixture is smooth, slushy, and evenly combined. You'll hear a consistent hum, and the mixture should look thick and icy.
Stop the blender and check the consistency—if it's too thick, add a splash more milk and blend briefly again, or add more ice for a thicker texture.
Pour the chocolate slushie into glasses, watching it flow in thick, fudgy ribbons.
Serve immediately with a straw or spoon, and enjoy the cold, creamy, chocolatey treat.