Chocolate Phosphate Soda

Combine cocoa, sugar, water, and salt in a small saucepan and bring to a boil over medium-high heat, stirring frequently. Keep your eye on the syrup as it approaches boiling because it's prone to boil over. Immediately reduce the heat to low and keep the syrup at a steady simmer until it reduces enough to coat the back of a spatula, about 10 minutes.

Remove from heat, cool to room temperature, and store in the refrigerator.

Place seltzer, acid phosphate, and chocolate syrup in a highball or other tall glass. Stir to combine. Top each glass with ice to fill and serve immediately.