1 - 2tbspagave syrupoptional - only if you want to add extra sweetness
Extra milk if needed
Place the Nutella in a bowl. Add warm milk / half and half a little at a time and whisk until you have a smooth sauce (¼ cup milk is the minimum amount required, add more milk if you like a thinner sauce). Set aside until needed.
Nutella Frappuccino / Nutella Coffee Slushie
Place the milk / half and half and Nutella in the blender. Blend for a few seconds until the Nutella is completely mixed with the milk. (Nutella and milk are blended first because once you add the ice cubes, it'll be harder to completely dissolve the all of the Nutella).
Next, add the coffee ice cubes (and agave and vanilla, if using) and blend on high speed until the ice cubes are completely crushed and you have a slushie-like consistency. Add extra milk ONLY if required to help crush the ice better.
Drizzle some Nutella sauce inside the serving glass (this will also add some extra sweetness, if necessary). Pour your Nutella frappuccino into the serving glass.
This can be served just as is with a straw, or top it up with whipped cream and another drizzle of Nutella sauce. Enjoy!