In a medium saucepan, stir together the blueberries, water, maple syrup and salt.
Bring to a simmer over medium heat and simmer for 3-5 minutes or until it’s thickened quite a bit like blueberry sauce.
Stir in the vanilla extract.
Transfer to the blender (or place on a plate or whatever with a large surface area if you want it to cool quickly and place in the fridge or freezer for a few minutes) and let cool for about 15 minutes. You don’t want it to melt the ice cream.
While it’s cooling, prepare the rim of the milkshake glasses and the whipped cream. Pour sanding sugar into a shallow bowl. Brush some maple syrup on the rim of your glass with a small pastry brush, or dip the rim into a shallow dish of maple syrup. Immediately dip the rim of the glass into the sanding sugar. Tap to remove excess sugar and place in the freezer for a few minutes to help the sugar adhere better.
Add the heavy cream to a bowl. Begin by whipping the cream at medium speed. Once it starts to thicken, increase the speed to high. Continue whipping until the peaks stand up straight. Place in the fridge while you continue with the milkshake.
Once the blueberries have cooled enough, add 3/4 cup (90 ml) milk and the ice cream to the blender.
Blend until totally smooth and only tiny blueberry skin flecks remain. Add the remaining 1/4 cup of milk, if needed.
Top with whipped cream and a meringue ghost.