Pour the milk into your blender, then add the cocoa powder, sugar, vanilla extract (if using), and espresso shot. Blend on low until the cocoa and sugar start dissolving, and the mixture is smooth and slightly frothy.
Add the ice cubes to the blender, then secure the lid tightly. Blend on high until the ice is broken down and the mixture becomes thick, slushy, and smooth with a glossy appearance. This may take about 30-45 seconds.
Stop the blender briefly and check the texture—if it’s too icy or chunky, give it a quick pulse. If it’s too soft, add a few more ice cubes and blend again for a few seconds until it reaches your preferred consistency.
Once the mixture is smooth, velvety, and icy but scoopable, pour it into glasses. The drink should look glossy with a deep chocolate color and a thick, slushy consistency.
Optionally, garnish with a sprinkle of cocoa powder, a few chocolate shavings, or a small scoop of whipped cream for presentation. Serve immediately with a straw or a spoon for scooping.