Add the blueberries, granulated sugar, and 1/2 cup water to a medium saucepan. Stir gently to coat the berries with the sugar. If using frozen blueberries, they may clump at first; they will loosen as they warm.

Place the saucepan over medium heat. Cook, stirring occasionally, until the mixture begins to bubble around the edges and the blueberries start to burst and release their juices.

Stir in the lemon juice, lemon zest (if using), vanilla extract, and a pinch of salt.

For a smoother syrup, lightly mash some of the blueberries against the side of the pan with the back of a spoon or a potato masher, leaving some whole for texture if desired.

In a small bowl or measuring cup, whisk together 1–2 tablespoons cornstarch with 2 tablespoons cold water until completely smooth and lump-free.

Reduce the heat to low. While stirring the blueberry mixture, slowly pour in the cornstarch slurry. Continue stirring for 2–4 minutes, until the syrup thickens and turns glossy. If it is still too thin, mix another small amount of cornstarch with cold water and add gradually.

Remove from heat. Remember the syrup will thicken more as it cools. Taste and add a bit more sugar for extra sweetness or a splash more lemon juice for more brightness, if desired.

For a very smooth syrup, pour the hot mixture through a fine-mesh strainer into a heat-safe bowl or jar, pressing on the solids to extract as much liquid as possible. Reserve the pulp for yogurt, oatmeal, or smoothies.

Let the blueberry syrup cool slightly before serving. It will continue to thicken as it cools. Serve warm over pancakes, waffles, French toast, ice cream, cheesecake, yogurt, or stir into sparkling water for a blueberry soda.