Add the milk, cocoa powder, sugar, and heavy cream (if using) into the blender. Secure the lid firmly.
1 cup milk
Blend on high until the cocoa and sugar are fully dissolved, and the mixture is smooth and frothy, about 30 seconds.
Add the ice cubes to the blender.
1 cup milk
Blend again on high speed until the ice is crushed and the mixture is thick, icy, and slushy, approximately 1 minute.
Stop the blender and check the texture; it should be creamy, icy, and smooth when scooped.
Pour the chocolate slushie into glasses and serve immediately with optional toppings like whipped cream or chocolate shavings.