Place the raspberries in a pan and cook over high/medium heat for 2-3 minutes, mashing up with the back of a large spoon and stirring all the time. Transfer the mixture into a fine-mesh strainer, place over a bowl and press and stir for a few minutes to remove the seeds. Discard the seeds.
Place the raspberry sauce back in the pan, add the sugar, cacao/cocoa and corn flour, bring to boil and simmer for 2 minutes stirring with a whisk all the time. Gradually add the milk, whisking all the time, bring to boil again, then simmer for 2-3 more minutes. Remove from the heat, stir in the butter and serve (or wait for the sauce to cool down first). If the sauce gets too thick after a while just add a bit of milk.
Raspberries: Use either fresh or frozen.
Texture: Use a whisk to stir the sauce as it's cooking to prevent lumps forming. Use either cocoa or cacao though bear in mind cacao is a tiny bit grainier (check out this collection of 27 everyday healthy cacao recipes).
Serving: Enjoy hot or cold. The sauce is likely to thicken after a while so just add a bit of milk to get the consistency you want.
Storing: You can refrigerate leftover sauce, covered, for up to 3 days.