Combine blueberries, sugar, and water in a small saucepan and bring to a boil over medium heat, stirring occasionally, about 3 to 4 minutes.

Reduce heat to low and simmer the mixture for 5 minutes, until the berries break down completely.

Remove from heat and stir in lemon juice and vanilla extract, if using.

Strain the syrup through a fine mesh strainer into a clean jar, pressing gently on the solids to extract all liquid.

Allow the syrup to cool to room temperature, then seal and refrigerate for up to two weeks.

Heat the espresso machine or moka pot and pull two shots of espresso (about 2 ounces) into a tall mug.

Stir 2 tablespoons of cooled blueberry syrup into the hot espresso until fully combined.

Steam and froth 10 ounces of cold milk using a steam wand or milk frother until it reaches 150 to 155°F (65 to 68°C) and has a smooth microfoam texture.

Pour the steamed milk into the mug, holding back the foam, then top with a quarter inch of milk foam for garnish.