In a medium saucepan, whisk together cocoa powder and granulated sugar until well combined.
Gradually pour in cold water, whisking until the mixture is smooth and free of lumps.
Place the saucepan over medium heat and stir constantly until the mixture begins to boil.
Once boiling, add kosher salt and vanilla extract, stirring well to combine.
Reduce the heat to low and let the syrup simmer for about 5-10 minutes, stirring occasionally.
Remove from heat and let the syrup cool completely before transferring to a storage container.