In a medium bowl or glass measuring cup, combine the water, cornstarch, cocoa powder, and sugar. Whisk to combine. Heat in the microwave in 30-second to 1-minute increments until thickened, stirring between heatings, 2-3 minutes in total.
½ cup water, 1 ½ tbsp cornstarch, 3 tbsp Dutch-process cocoa powder, ½ cup granulated sugar
Once thickened, add the butter and vanilla extract. Whisk until smooth and enjoy immediately or store in the refrigerator for up to 1 week. Heat slightly before serving.