Add the milk, cocoa powder, sugar, and vanilla extract (if using) into your blender. Blend on low speed at first to combine the dry ingredients with the milk, then increase to high and blend until the mixture is smooth and frothy, about 30 seconds.
Add the ice cubes to the blender. Secure the lid firmly and blend on high speed until the mixture is thick, frosty, and spoonable, about 1 minute. The ice should be fully cracked and incorporated, giving the slushie a velvety appearance.
Stop the blender and check the consistency. If it's too thick to blend smoothly, add a splash more milk; if too thin, add a few more ice cubes and blend briefly again.
Once the mixture is evenly ice-cold, frosty, and smooth, pour it into chilled glasses. The slushie should have a thick, icy texture that holds its shape when served.
optionally, top with whipped cream, chocolate shavings, or a drizzle of chocolate syrup for extra indulgence. Serve immediately with a straw or spoon to enjoy its icy, velvety goodness.
