- Pour the milk into the blender and add the cocoa powder, sugar, and vanilla extract. 
- Blend on low speed until the cocoa and sugar are dissolved and the mixture is smooth, about 30 seconds. 
- Add the ice cubes to the blender and secure the lid tightly. 
- Blend on high speed until the ice is fully crushed and the mixture is thick and slushy, about 1 minute. 
- Stop the blender and check the consistency; it should be creamy and icy with a velvety texture. 
- Pour the chocolate slushie into glasses, garnish with additional chocolate shavings or a swirl of whipped cream if desired, and serve immediately.