Combine frozen blueberries, brown sugar, and water in a small pot and heat over medium heat for 10-15 minutes, stirring occasionally, until the berries break down and release their juice. The mixture will become dark and syrupy as the sugar dissolves and the berries soften. Once the berries are completely broken down, remove from heat and let cool slightly, then mash the berries against the side of the pot with the back of a spoon to release any remaining juice. Strain the mixture through a fine-mesh sieve into a jar, pressing gently on the solids to extract all the syrup while leaving the pulp behind.

Fill a tall glass with ice, then add 2 tablespoons of the blueberry syrup from Step 1, stirring briefly to coat the ice. Pour in the full fat oat milk, then top with 2 shots of espresso. I like to pour the espresso last so it creates a beautiful color gradient as it swirls into the milk and syrup. Stir well to combine all the flavors, add the vanilla extract and salt for depth, and serve immediately with a spoon or wide straw for sipping.