Combine the cocoa powder, sugar, sea salt, vanilla extract, and milk in a blender. Blend on low until the cocoa and sugar dissolve completely, creating a smooth, chocolatey base.
Add a generous handful of ice cubes to the blender. Continue blending on high until the mixture becomes thick, frosty, and scoopable, with a texture similar to a soft-serve ice cream.
Stop blending and check the consistency. If it's too runny, add more ice and blend again until the desired icy texture is achieved.
Pour the chocolate slushie into glasses, ensuring it's thick and visually appealing with a frosty surface.
Optional: Garnish with a sprinkle of sea salt or a drizzle of chocolate syrup for extra flavor and presentation.
Serve immediately with straws or spoons, enjoying the rich, icy treat on a hot day.