Measure out the chilled leftover brewed coffee and pour it into your blender.
Add the sweetened condensed milk to the blender, which will give the slushie a rich sweetness and creaminess.
Sprinkle a pinch of sea salt into the mixture; this enhances the flavor and balances the sweetness.
Drop in the ice cubes, ensuring they're evenly distributed for smooth blending.
Secure the lid on the blender and blend on high speed until the mixture is smooth, thick, and resembles a marbled, slushie consistency. This may take about 30-60 seconds.
Check the texture; it should be icy yet scoopable, with a velvety appearance. Pour into glasses.
Optionally, garnish with a drizzle of condensed milk or a few coffee beans for presentation.
Serve immediately with a straw or spoon, and enjoy this refreshing coffee treat.
