Start by making the simple syrup: add sugar, water, vanilla, and rosemary to a saucepan and bring to a boil; then, reduce to a simmer and simmer for 15 minutes. Allow syrup to come to room temperature before using.
For the cocktail: fill 2 glasses with ice then set aside. Next, combine cold brew, syrup and milk in a cocktail shaker; shake to combine, then pour into a glass. Last, pour Bom Bom overtop and stir with a spoon. Alternatively, you can add Bom Bom to the cocktail shaker and combine it that way.