Add the milk, cocoa powder, sugar, chili powder, vanilla extract (if using), and ice cubes into the blender.
Secure the lid on the blender and blend on high speed until the mixture is smooth, thick, and frosty, about 30-40 seconds.
Stop the blender and check the texture; if it’s too thick, add a splash more milk and pulse a few times to loosen.
Pour the slushie into chilled glasses, allowing the icy texture to settle for a few seconds.
Garnish with a sprinkle of chili powder or a dollop of whipped cream for presentation, if desired.
Serve immediately with a straw and enjoy the cool, spicy chocolate flavor.