Step 1: Create Thickened Chocolate Base
In a small bowl, mix cornstarch with 1/2 cup cold water until smooth.
In a saucepan, heat remaining 2 1/2 cups water and sugar over medium heat, whisking until dissolved.
Gradually whisk in cocoa powder until fully combined.
Pour in cornstarch mixture while whisking constantly.
Cook for 3-4 minutes until slightly thickened (should coat the back of a spoon).
Step 2: Add Chocolate & Enhance Flavor
Remove pan from heat and add chopped chocolate. Let sit for 1-2 minutes, then stir until fully melted.
Stir in vanilla extract and salt if using.
Strain through a fine-mesh strainer if needed for extra smoothness.
Step 3: Freeze Without Ice Cream Maker
Pour mixture into a shallow metal baking dish. Let cool at room temperature for 30 minutes, then cover with plastic wrap.
Freeze for 2 hours until edges start to harden.
Remove and beat with a hand mixer or whisk vigorously to break up ice crystals.
Repeat mixing process every hour for 2 more times.
Freeze for 4-6 hours or overnight until firm.