Pour the milk into your blender, creating a smooth base for the drink.
Add the unsweetened cocoa powder and granulated sugar directly to the milk.
Blend on high for about 15-20 seconds until the mixture is well combined and slightly frothy.
Pour in the cooled espresso or strong brewed coffee into the blender, and add a pinch of sea salt if using.
Add the ice cubes to the blender, ensuring they are evenly distributed.
Blend again on high for 30-40 seconds until the mixture is thick, slushy, and smooth with a frosty appearance.
Turn off the blender and check the consistency; it should be thick and cold with tiny ice crystals visible.
Pour the frozen chocolate espresso slushie into glasses and serve immediately for the best texture.
Garnish with chocolate shavings or a sprinkle of cocoa powder if desired, then enjoy!