Measure out the milk, cocoa powder, sugar, vanilla, and ice cubes, placing them nearby for easy access.
Add the milk, cocoa powder, sugar, and vanilla extract into the blender and secure the lid.
Start blending on low speed, gradually increasing to high, and blend until the mixture is smooth and the ice is finely crushed.
Check the consistency; if it's too thick, add a splash more milk and blend briefly to loosen. If it's too thin, add a few more ice cubes and blend again.
Stop the blender and observe the creamy, slushy mixture with a glossy chocolate appearance.
Pour the chocolate slushie into glasses using a spoon or spatula, ensuring a generous, frosty portion.
Serve immediately with optional toppings like whipped cream or chocolate shavings for extra indulgence.