Measure milk pour it into a heavy-bottomed saucepan and heat it. Stir it until it starts to boil. Reduce heat to low. Pour in the agave syrup. Put in vanilla extract too. Now leave saucepan with the milk in low heat for about an hour stir occasionally. The purpose is to reduce the amount of the milk in half. When it's reduced and the amount is about 2 cup / 500ml turn off the heat and leave in to cool a little bit before blend it together with the butter. Whisk butter in. Mix until smooth. Store it in the fridge. Use it up within a week.