Combine the cocoa powder, caster sugar and water in a small pot. Bring it to a boil over medium-high heat, whisking constantly to dissolve the sugar and cocoa powder.
Remove the pot from heat and stir in the butter. Let the sauce cool before transferring it to a serving container. The sauce will thicken slightly when cooled.
Make ahead: Keep the sauce covered in the refrigerator for up to 1 week. Warm it up in the microwave or in a pot and whisk till smooth before using.
NOTES
Use the best-quality cocoa powder you can afford - my favourite brand at the moment is Valrhona.
Feel free to add more sugar if you desire a sweeter sauce.
For a richer and thicker sauce, substitute water with milk.