In a small saucepan over medium heat, combine the blueberries, honey, vanilla, and water. Simmer 8–10 minutes, until the berries burst and the liquid reduces into a syrup.
Gently mash the berries with the back of a spoon. Strain out the skins for a smooth syrup, or leave as is. Cool, then chill in the fridge.
In a frother, small blender, or mason jar, combine blueberry syrup, half and half, and a splash of vanilla. Blend or shake until light, airy, and frothy.
Fill a glass with ice. Add 1–2 tablespoons blueberry syrup to the bottom, pour in the milk, then slowly add the freshly brewed double espresso.
Finish with a generous spoonful or two of blueberry foam on top.
Notes
Make the syrup the night before. A mason jar works for the foam — just shake.